BOTANY I

"Live in each season as it passes; breathe the air, drink the drink, taste the fruit, and resign
​yourself to the influence of each."
- Henry David Thoreau

Introduction

Botany is a branch of biology concerned with the study of plant life, including plant structures, properties, and processes. It also covers the study of plant diseases and interactions with the environment, including their critical contributions to world biodiversity. Botany is a great jumping-off place for those who want to delve deeper into subjects like agriculture, horticulture, biochemistry, and forestry. The study of botany is important because it is the science that investigates how all life on Earth functions. ​The planet we have the pleasure of living on is covered in life; much of it plant life. Plants play essential roles in many of the natural systems that exist on earth, converting energy from the sun into food, drawing crucial minerals from the soil beneath them, and providing shade, refuge, food and protection for all living around them.

​Quick Question:
What's the difference between Horticulture and Botany?

Both areas of study are concerned with plants first and foremost. However, horticulturalists are more likely to be interested in the real-life application of topics concerning plant life, like growing and cultivation of crops and landscape plants. Botanists, on the other hand, are more interested in theoretical study, and are concerned with a wider range of organisms, including fungi and bacteria.
Gardeners, or horticulturalists, work with plants on a regular basis, so it is important to have a basic understanding of botany to answer the many questions that will arise as plant life is cared for, such as:
The questions that exist about plant life are endless. Thankfully, the field of botany is around to answer them for us. Through becoming familiar with the category, classification, reproductive and developmental aspects of the plants we work with, we can observe our gardens in a whole new way. Understanding fundamental principles of plant physiology and botany, combined with skill and intuition in employing them, ensures maximum use and enjoyment of plants.

The Plant Kingdom

A plant is a living organism, usually containing chlorophyll - including trees, shrubs, herbs, grasses, ferns, and mosses - that typically grows in a permanent position, absorbing water and inorganic substances through its roots and synthesizing nutrients in its leaves by photosynthesis. Unlike animals, which stop growing once they are mature, many plants can grow and reproduce exponentially. ​Plants are found in every climate worldwide, except for totally arid or frozen areas. They range in size from giant Sequoia trees to plants smaller than a grain of rice. Originally, all plants were aquatic, and over time evolved to grow on land, assisted by fungi to absorb nutrients from soil. The first plants to colonize land masses were ancestors of modern-day mosses, liverworts and hornworts. The kingdom Plantae is generally divided into two groups: nonflowering plants and flowering plants. Nonflowering plants reproduce from spores instead of flowers, and include liverworts, mosses, hornworts, lycopods, ferns, and gymnosperms. Flowering plants are also called angiosperms, and this category of living organisms is further divided into magnoliids, monocots, and eudicots. As gardeners, we are mostly interested in two groups of plants: Angiosperms and Gymnosperms.
Quick Question: What is NOT a plant?
Fungi, lichen, and algae are not technically plants, although they do look similar. These living beings are more closely related to animals, and cannot produce their own food. They instead rely on carbohydrates that are formed by plants for energy.
Fungi
Lichen
Algae

Angiosperms and Gymnosperms

Angiosperms
​This is the most diverse group of plants, representing around 80% of all plant life on earth. They are vascular seed plants in which an ovule (or egg) is fertilized within a hollow, enclosed organ called an ovary. This ovary is typically contained within a flower, which carries either male or female reproductive organs (or both). Angiosperms are found in almost every habitat worldwide. There are an estimated 352,000 species of angiosperms on the planet. These life forms are vital to human health, agriculture, industry, and ecological balance, making them an indispensable part of both the economy and the environment around us. The plant genera listed below are all angiosperms.
Gymnosperms
​Gymnosperms are plants that have seeds which develop on the surface of scale or leaf-like appendages of cones or at the end of short stalks. They do not produce flowers. They are members of a more ancient group of plant life, and there are about 1,000 species found on Earth. They tend to be plants like trees and shrubs. The genera below are common examples of Gymnosperms.
Angiosperms and gymnosperms encompass many of the plants we as gardeners will be growing in our landscapes, but we are also interested in plants like ferns and mosses, depending on what type of garden we are cultivating. These plants are categorized in the same nonflowering plant group as gymnosperms, and they have different reproductive and evolutionary stories. They will be covered in later modules.

Plant taxonomy and Classification

Plants are identified according to a system devised by Swedish botanist Carl Linnaeus (1707-1778). This system used two-word names in Latin and italics, and grouped plants together according to shared characteristics. Historically, this classification system was based on a plant's physical characteristics (like flower structure) or biochemistry, and was more often than not speculative or subjective. In these modern times, botanists use genetic testing and resulting evidence to classify plants and their relationships to one another. This more accurate way of categorizing plants has led to some interesting discoveries in the genetic profiling of plant life.

The International Code of Botanical Nomenclature exists to organize the many species of plants, and currently consists of seven ranks: Kingdom, Division, Class, Order, Family, Genus, and Species. A good way to remember this system is the phrase:
King  David   Came   Over   For   Good   Spaghetti

Let's look at an example to illustrate what this system looks like using a common tree, the Valley Oak:

​Kingdom: Plantae (plants)
Division: Magnoliophyta (flowering plants)
Class: Magnoliopsida (dicotyledons)
Order: Fagales
Family: Fagaceae (Beech family)
Genus: Quercus (Oak)
Species: Quercus lobata (Valley Oak)
​
The way to write the scientific name of the Valley Oak tree is Quercus lobata. Scientific names of plants and animals are written out using a binomial (or two-name) system. The first name refers to the genus and the second is the species name. Scientific names are always italicized, and the first letter of the genus name is always capitalized. If there is a specific cultivar or a variety of a species, this name is listed after the scientific name, is not italicized, and is usually enclosed in single quotation marks, as shown in these variations of Wisteria:
Wisteria floribunda 'Caroline'
Wisteria floribunda 'Black Dragon'
Wisteria floribunda 'Shiro Kapitan'
Gardeners are concerned mainly with the family, genus and species of plants, as well as the subspecies, variety or cultivar. In the Plant Pages module, you will become more familiar with how these botanical names are used and learn how to identify many different plants using this classification system.

Common Specific Epithets

In identifying plants, it is good to get a basic overview of commonly used Latin- and Greek-based descriptive terms that are used in scientific names. Many of these will refer to the appearance of flowers or leaves, and some will refer to their native region. The page linked below contains a list of some of the more common epithets used in naming species of plant life.

alternative ways to classify plant life

In addition to the scientific classification and naming system used by botanists and horticulturalists, there are a few more ways that plants can be classified. The most common include categorization according to growth habit, structure or form, leaf retention, climatic adaptation, and use by humans. The first is an important distinction that is made within the Angiosperm group of plants.

Monocots and Dicots
​There is an important distinction between species of flowering plants that indicates a few things about their growth and interior makeup. Flowering plants (angiosperms) are divided into Monocots and Dicots. Put simply, monocotyledonous plants have one cotyledon and dicotyledonous plants have two cotyledons. The cotyledons are the bottom-most leaves on a stem; and are also called seed leaves. They emerge from the seed, allowing the seedling to immediately start photosynthesizing and growing. There are some differences between the two groups that are apparent both inside the seeds as well as in the structure of mature plants. ​As mentioned above, monocot plants have one cotyledon (seed leaf) within their seed, and dicot plants have two. ​Monocots have fibrous roots, and dicots have taproots. ​Monocots have a scattered pattern on their interior, whereas dicots have a ringed pattern to their vascular bundles.
​Leaf venation (which we cover farther down in this module) differs as well; in monocots, leaves have parallel veins, and in dicots, leaves have palmate venation. ​​Flowers produced by monocot plants have petals in multiples of 3, while dicot flowers have petals in multiples of 4 or 5. Pollen grains differ between the two flowering plant classifications, as well. In monocots, pollen has a single pore (it is monosculate), and in dicots, pollen has three pores (trisculate).
​Growth Habit
While planning a garden, it is important to learn the growth habit terms used to classify plants: annual, perennial, and biennial. Annual plants complete their full life cycle in one year. These plants sprout from seed, grow, flower, and reseed within 12 months' time. Perennial (either monocarpic or polycarpic) plants, on the other hand, survive multiple seasons before fading. Perennial monocarpic plants have just one instance of flowering and setting seed, and then the parent plant will die. Perennial polycarpic plants, on the other hand, are those that flower and produce seeds every year. Perennial plants can also be those that survive in dormancy underground as a storage root, and re-sprout from the ground when the climate is conducive to herbaceous growth. Biennial plants, by contrast, require two years of growth before completing their life cycle. Plants in this category will grow leaves and shoots in their first year and then in their second year they will produce flowers, commonly requiring a cold period after the first year where the plant enters dormancy. Check out the examples below for more information on these various life cycles:
Watermelon is an example of an annual plant; to have more than one year of harvest, new plants must be grown each year.
Agave is an example of a perennial plant that lives for a number of years before flowering, and then dies. Plants that follow this pattern are said to be monocarpic.
A majority of perennial plants will survive for many years, with multiple seasons of flowering and setting seed. These plants are said to be polycarpic. A good example is Hibiscus.
Biennial plants produce leaves and shoots in their first year and flower in their second before dying. A great example of a biennial plant is Hollyhock.
Some plants are annuals in some areas, and perennials in others. For example, in mild-winter areas, Tomato plants are considered perennial plants, while in cold-winter areas, they are grouped with annuals.
Annuals are popular because although they only live for a year, they produce beautiful flowers. Many gardeners create beds for annual plants that are switched out each spring.
Structure or Form
​Plants can be classified according to the basic size, structure, or form they take. This way of classifying plants varies widely, but is very useful in planning landscapes. The two main groups are woody (plants with hard, fibrous stems) and herbaceous (plants that have tender, green stems). Plants are also grouped according to their general form; for example, vines, shrubs, and trees. These groups are further divided according to their shape. Shapes can be described as open, vase, upright, weeping, pendulous, spreading, or irregular, just to name a few.

​Leaf Retention
Perennial plant species generally fall into two categories: deciduous or evergreen. Deciduous plants lose their leaves yearly for a period of time during the cooler months of the year. Evergreen plants do not go through this dramatic loss of leaves each year. Evergreen plants do lose leaves, but they shed them on a periodic basis, and not all at one time. Evergreen plants are further divided into broadleaf and needle-leaved species.
Deciduous plants lose their leaves for periods of time during the year when the plant goes into dormancy.
The leaves on deciduous plants commonly turn colors before falling.
Evergreen plants do not lose their leaves, providing a green landscape year-round.
Some evergreen trees have needle-like leaves that are long, thin, and usually stiff. Conifers have needle-like leaves.
Other evergreen species have wider leaves, and are referred to as broadleaf evergreens. An example of a broadleaf evergreen is Azalea.
Some trees are considered briefly deciduous, because they hold onto their leaves longer into the cold season and/or sprout leaves earlier than other deciduous plants. A great example is Chinese Elm.
Climatic Adaptation
In this classification style, plants are grouped by the environment to which they have adapted best to survive. There are four general categories of plants: terrestrial, aquatic, epiphytic and lithophytic. Terrestrial plants grow in, on, or from land. Most of the plants in the garden are terrestrial plants. Aquatic plants (also called hydrophytes or macrophytes), as the name hints, grow in salt or fresh water. A good example of an aquatic plant is Nymphaea (Water Lily). Epiphytic plants are plants that receive their nutrients and water from the air and use their roots to anchor themselves to another plant or structure. Examples of epiphytic plants are Orchids and Air Plants. Lithophytic plants are those that grow on rock formations, either on the rock's surface or tucked into crevices. They hydrate using water that runs across the surface of the rock, and get nutrients from nearby decaying matter (including their own).

Another way to divide plants by their climatic adaptation is according to the minimum temperature they can tolerate. Perennial plants are divided into three groups: tropical, subtropical and temperate. Tropical plants are injured severely or killed when temperatures remain near freezing (32 degrees Fahrenheit). These plants are commonly grown in cool-weather winter areas indoors as houseplants, but can sometimes survive the winter in sheltered, outdoor areas. Subtropical plants can tolerate short periods of freezing temperatures (and in some cases a bit below freezing). Temperate plants can take prolonged periods of time exposed to below freezing temperatures. Annual plants are also divided according to the temperatures they can withstand. Cool-season (also called 'hardy') plants and crops can tolerate some short-term freezing temperatures. They grow best in daytime temperatures of between 55 and 75 degrees Fahrenheit. Those that go dormant, are killed, or are seriously injured by freezing temperatures are called warm-season or tender crops. These tender crops grow and produce best when daytime temperatures stay between 65 and 95 degrees Fahrenheit.

Use
The last main method of classifying plant life is by how they are used in the landscape, and whether their parts are consumed by humans. Plants that are grown for their aesthetic beauty or environmental enhancement qualities are called ornamentals. Crops, on the other hand, are plants grown primarily for their edible parts. These include nuts, fruits, herbs, roots, and vegetables. A new category of plants, edimentals, is growing in popularity, and includes plants which are grown for both their ornamental value and have the added bonus of being edible.

The Basics of Botany & Plant Structure

Now that you've gotten a basic understanding of the plant kingdom and how we classify plants, it's time to familiarize yourself with the basics of plant botany and structure. This next section will cover different plant parts and their functions. Botany as a field encompasses a wide range of sub-topics, but we will be mainly looking at three specifically: Plant Structure, Reproduction, and Growth. These three topics are crucial to understanding the plant life you have in your garden, including how to best grow plants, how to eradicate and prevent issues associated with watering and sun exposure, and how to form your plant into its healthiest and most attractive shape. The basic structures that make up a plant are seeds, meristems, roots, stems, buds, flowers (including their resulting fruit), and leaves.​​​

seeds

Seeds contain embryonic plants in a dormant state of development along with food reserves which sustain them through germination time (until they have produced leaves and begun photosynthesis). Most seeds have a seed coat that protects the embryonic plant. Seeds are produced by mature plants through reproductive activities, and can be released on their own or stored within fruits. Seeds, once released from their parent plant and located in a conducive environment, will begin to sprout, eventually growing into their own individual plant. Seeds come in all different shapes, sizes. and colors, and will be covered in more detail in the Seeds module.
​

meristems

These specialized, formative tissues are made up of small cells capable of dividing indefinitely and giving rise to similar cells or to cells that differentiate to produce tissues and a variety of organs. The two most common types are apical meristem and lateral meristem. Apical meristem is found in buds, shoot (stem) tips, and root tips, and is responsible for the increase in length of these plant parts. Lateral meristem, on the other hand, is responsible for increasing the thickness of plant parts and is also called the cambium. These meristematic areas are just a few cells deep and can produce shoots (vegetative growth) or flowers (reproductive growth), depending on when and where the meristem is active. These meristematic areas are referred to as 'sinks' because they receive priority for available food and minerals. 

roots

Roots are plant parts that bear other roots and stems, but they cannot produce leaves. They have a characteristic structure, including a growing tip, or root apex. Roots have several functions. They anchor a plant in the ground, absorb nutrients and water from the soil around them and support bacteria or fungal hyphae, which work with the roots to the benefit of both organisms. Roots are important to think about while redesigning and planning a garden. Depending on a plant and its root system, attention may need to be paid to issues like compacting the soil around the base of a plant, avoiding contaminating clean soil, how you are trenching around roots, and whether or not to bury a containment system to send roots farther down into the ground before they spread outward.
Root Systems
Roots are formed in two main ways during the development of a plant. Primary root systems have branches derived from the hypocotyl, the portion of the embryo below the seed leaves (cotyledons). They are the first root structures formed. In monocots, the primary root system does not develop well; instead an adventitious root system forms, in which roots arise from stem or leaf tissue. Root hairs form on roots, and serve to gather water and food for the plant. These hairs are very small, only a few cells wide, and live for a few days before falling off the root. New hairs form farther down the root as it grows. These hairs greatly increase the surface area of the roots, and in doing so, increase the amount of water and nutrients a plant has access to. Water and its contained minerals are absorbed into cells through osmosis and are transported through the 'cortex' of the hair into the vascular system of the plant.

The two main types of root systems are referred to as taproots and fibrous roots. A taproot consists of a single, large central root with smaller side roots, and fibrous root systems consist of networks of many small roots. Fibrous root systems are found in all ferns, in many grasses, and in other flowering plants. Some trees will produce a taproot initially and then develop a fibrous root system as they age. Fibrous roots have extensive branching structures that create a spreading network to anchor the plant firmly in place and help the plant to find water and essential minerals from a wider area of soil. These roots have better access to oxygen, which is found at a higher concentration closer to the soil surface. They can also go a long way in helping to prevent erosion of sloped areas.

Taproots are found in plants like root vegetables, flowering plants and some genera of trees. These roots have a dominant primary root with smaller side roots, and grow deep into the soil. Some plants will begin with a taproot, which falls off eventually, causing the remaining roots to form a fibrous root system. Taproots will swell as they collect nutrients and water from the soil, and store food in the form of carbohydrates. This food is then used when the plant flowers. This is why vegetables like beets and carrots are commonly harvested before the plant flowers. After blooms fade, the taproot becomes woody and unpalatable, having used up its stored carbohydrates during the flowering and seed producing processes. A benefit of having a taproot is that the plant can regrow easily from just a piece of the root that is left in the ground. This is why it is so hard to remove weeds that have taproots; if you've ever had a Dandelion (Taraxacum officinale) problem, you have personal experience with this at times frustrating quality.
Fibrous root system.
Taproot system.
As stated above, one of the main functions of any root system is to anchor a plant in place in the soil. For many plants, this task is completed solely underground. In areas where the soil is shallow, however, roots have evolved to form elaborate above-ground systems. These supportive root systems are divided into three categories: Buttress, Prop, and Stilt. In buttress systems (also called Plank Root systems), roots flare out from the trunk of the plant and are part of the primary root system. Prop roots are aerial roots that grow out from the stem or trunk. New props form as the plant increases in size. Stilt supportive root systems are formed by branches that bend down towards the ground and develop into roots. These roots are great at anchoring trees and other plants that are grown in areas with strong tidal flows.
Buttress Roots
Prop Roots
Stilt Roots
There are a few other types of specialized roots worth mentioning that have evolved to assist plants with better absorbing the nutrients and water they need. These include contractile roots, aerial roots, and roots that simply serve to anchor and do not transport water or minerals. Contractile roots can shift the position of a plant's depth, and are commonly found in plants with storage roots like taproots, bulbs, corms, or rhizomes. These roots will pull plants deeper into the soil as they mature by shrinking and extending themselves. Contractile roots ensure that plants have the support they need to flower (and not topple over as they do so). These roots are also useful for these plants because they pull the main root lower into the soil where it is cooler so that plants can go dormant during hot or cold periods of the year. Epiphytic plants live resting on the structure provided by other plants like trees and shrubs. These plants need roots that hold them firmly in place, and aerial roots do the trick. These roots emerge along stems, and cling to any surface they can. They absorb water through mist, fog, and rain, and in some cases can become green and assist in photosynthesis.
Hyacinth plants have underground bulbs and contractile roots that pull the bulb farther into the ground in summer after flowering so the plant can survive the cold winter months and bloom the following spring.
Monstera plants have aerial roots that sprout from stems. These aerial roots absorb water that is in the air.
Air plants, members of the genus Tillandisa (which is in the Pineapple family - Bromeliaceae), can absorb moisture from the air through specialized scales on their leaves. These plants do have roots, but they serve only to anchor the plant, they do not absorb moisture or nutrients. Common 'hosts' for air plants include branches, rocks and twigs.
​
Pictured:Tillandsia tenuifolia, commonly called Narrowleaf Airplant, uses its roots to anchor to a structure. Its leaves have specialized scales on them that absorb water directly from the air.
All plant cells require access to oxygen if they want to survive. Roots of some aquatic plants have evolved hollow channels filled with a sponge-like structure called aerenchyma (air - ENG - kuh - muh). These structures run through the stems, roots, and leaves of aquatic plants and allow air to flow through the cavities from above the surface of the water down to the roots.
Equisetum plants have aquatic roots.
The interior of the stems, roots, and leaves of aquatic plants have spongy channels through which air flows.

​Another adaptation some water-dwelling plants have adopted is the formation of roots called pneumatophores. These roots are a bit like snorkels, helping roots that are growing in frequently submerged situations acquire oxygen. They develop in areas with shallow, coastal waters, and absorb air through lenticels on their roots.

stems and shoots

Stems are formed from the epicotyl, the portion of an embryo above the seed leaves (cotyledons). They usually have leaves, and their growing tip (shoot) has small stubs on its side, which develop into more leaves or flowers. Stems have several functions, acting as a support for the leaves (and holding them closer to the light), flowers, and fruits, and transporting nutrients to the leaves and synthesized carbohydrates to the roots. Stems can also be called halms (alternatively, haulms), or culms. In some plants, such as Cacti, the stems can also photosynthesize and store food. Stems can be above ground or underground, and form the main body of a plant.
​The cotyledon separates the hypocotyl from the epicotyl. The epicotyl is the part of the plant above the ground, and forms the stem. Stems are comprised of several parts. In the middle is the pith, a region of undifferentiated cells which together with the cortex, form a base for the other layers to grow in. Surrounding the pith is either a ring or a series of bundles containing cells that transport fluids. This is known as the vascular tissue. Outside of this is a layer of cells known as the cortex and then the epidermis, the skin of the stem, which is modified into bark in woody (dicotyledonous) plants.

On the three-dimensional and cross-sectional diagrams below, you'll see how complex eudicot stems really are. The vascular tissue in plants includes the phloem, cambium, and xylem. Phloem and xylem form in pipe-like structures that travel up through stems. Phloem is made of outward-facing cells that distribute food from photosynthetic tissue to other tissues. Xylem is made up of cells that are interior-facing and transports water and nutrients. Phloem and xylem are separated by a thin area of tissue called the cambium.
Stems are normally divided into nodes and internodes. Nodes are points along the stem where leaves sprout. Axillary buds (buds which will eventually grow into branches or tendrils) will sometimes form between the stem and leaf. Internodes are the spaces of stem in between the nodes.
Node
Internode
Axillary Bud
Quick Question: What is the difference between a stem and a shoot?
These two terms are often confused. The term 'shoot' refers to an instance of new plant growth, which can include stems, leaves, and flowers. The term 'stem' is used to reference simply the part of the plant that produces leaves and flowers. 
A plant stem.
New growth, or shoots.
Specialized Stem Terms & Stem Modifications
These stems have been specialized for food storage, asexual reproduction, protection, and increased photosynthesis. The list below includes these specialized stems, along with other descriptive terms for stems.
Acaulescent
Extremely short stems that grow to support plants that appear stemless. 
Branched
Aerial stems that support secondary branches.
Bud
An embryonic shoot with an immature stem tip.
Bulb
A short, vertical underground stem that has fleshy storage leaves attached.
Caespitose
The quality of stems that grow in a tangled mass on the ground.
Cladode
​A flattened stem that looks like a leaf and is specialized for photosynthesis.
Climbing
A stem that clings to or wraps around other plants or structures.
Corm
A short, enlarged underground storage stem.
Decumbent
The quality of a stem that lays on the ground and curls upward at the end.
Fruticose
The quality of having the form of a shrub.
Herbaceous
Non-woody stems that die back at the end of the growing season
Pedicel
A small stem that connects a flower to a peduncle.
Peduncle
A stem that supports an inflorescence or solitary flower.
Prickle
A sharpened extension of a stem's, root's, or leaf's outer layers.
Pseudostem
A  'stem' made out of the rolled bases of leaves.
Rhizome
A horizontal underground stem which has reproductive and storage capabilities.
Runner
A type of stolon that grows horizontally above-ground and produces roots at the nodes.
Scape
A type of stem that emerges from the ground and holds flowers but not leaves.
Spine
​A modified stem (or leaf stipule, or root) that is sharp. The term thorn is a synonym.
Stolon
A horizontal stem held close to the ground that produces rooted plantlets at its nodes and ends. 
Thorn
A modified stem with a sharpened point.
Tuber
A swollen, underground stem adapted for storage and reproduction.
Woody
The quality of a stem whose exterior is hard, formed with secondary xylem.
Sapwood
A woody stem, or the layer of secondary phloem that surrounds the heartwood and is active in fluid transport.
Quick Question: What is the difference between a stolon and a runner?
​A runner by definition is a thin, horizontal stem found above ground with a rosette of leaves at the end. A runner is very similar to a stolon; both structures allow the plant to move within its environment. Strawberries are a good example of a plant with runners. A stolon is a horizontally growing stem at ground level with leaves along its length (not just a rosette at the end, as in a runner) and adventitious roots that form at the nodes. White Clover has stolons. A sucker refers to a shoot that forms from a stolon.
Every Rose Has Its Prickle
Plants have evolved to develop defenses, arming themselves with pointed protrusions called thorns, prickles, and spines. These names are often used interchangeably, though they do not technically all refer to the same type of defense. Thorns are solitary, and sometimes will sprout and form their own branches. They appear at nodes, and are derived from stem tissue, contain vascular tissue, and can even bear their own leaves in some cases. They are stiff and woody. Examples of plants with thorns are Hawthorn, Citrus, and Firethorn. Spines are similar to thorns in that they contain vascular tissue (and look pretty similar), but they are created with leaf tissue instead of stem tissue. They are never branched, but can occur in clusters at nodes. Examples of plants with spines are various species of Cacti and Acacia.
Prickles develop as outgrowths of the epidermis and cortex of a plant and do not contain vascular tissue. They are not restricted to just stems, and can form on bark, leaves and even fruits. They occur all along stems, and are not restricted to the area around nodes. Many people mistakenly believe that the defenses commonly found on Rose (Rosa) plants are ​thorns, but they are actually prickles.
A Little About Tree Trunks
Tree trunks provide strong support for large masses of branches and leaves. They can grow to be very tall, and are difficult to remove without special heavy-duty equipment. Despite their height and supportive qualities, they amazingly do not need any extra supports themselves; they grow to stand alone. Trunks are the stems of trees, and develop bark on their exteriors to keep out invading insects, bacteria, fungi, retain moisture, and can even protect trees from fire damage. Some trees have evolved peeling bark to deter vines and epiphytic plants from taking up residence.

There are two important layers of bark, separated by two layers of cambium. These layers are relatively shallow compared to the width of the trunk, but if they are damaged, growth can be impeded or in extreme cases the tree can die as a result. As it ages, bark will split open under the force of outward growth, and create beautiful patterns, textures, and colors. These lenticels (openings or breaks in the bark) that form allow gases to pass through between living cells and the outside environment. The list below contains different types of bark and what trees they can be found on.
Lenticellate
​Prunus serrula
Scaly
​Pinus
Spiny
​Ceiba speciosa
Fissured
Liriodendron tulipifera
Trees and some other plants have resin canals on their interiors. These canals are surrounded by secretory cells that produce the resin. These canals ensure that the tree has an endless supply of resin, which is employed to defend the tree from attacking insects. Some trees only produce resin in response to damage, others have resin as part of their natural makeup. Resin heals tree wounds and is made of organic compounds produced by the tree. Economically, these resins are used in perfumes, varnishes and adhesives. Secreted resin gradually hardens into what we call amber, often trapping and preserving attacking pests.

buds

As stated above, buds are formed from meristem tissue and are pressed leafy shoots, flowers, or both. They can be dormant for a few months of the year to a few years before they become active. They are named according to where they appear on the plant. Buds at the tips of shoots are called apical buds (or terminal buds). The buds that are found along stems are referred to as axillary buds. There are also various types of specialized buds on certain plants, for example the 'eyes' on potato tubers are nodes that produce three buds each.

leaves

The leaf is the third basic organ of plants, and is usually defined by its position - that is, it originates as a small projection at the apex or tip of the stem, is attached to the stem, and in temperate plants there nearly always is a vegetative bud near where it is attached to the stem. Leaves are of utmost importance to a plant's survival. They are responsible for the vital processes of photosynthesis and transpiration and are usually green, though color can vary. Leaves can be either directly attached to a stem or attached by a short stalk called a petiole.
​Leaves come in all different sizes, shapes, colors, and textures. They can be either deciduous (they are discarded during certain times of the year) or evergreen (kept on the plant year-round). Leaves are expensive to produce in terms of energy, so evergreen leaves minimize resource demands in the long-term. However, if leaves are unlikely to survive certain climate changes, having deciduous foliage is more beneficial. Most leaves are filled with cells that harvest light for the purpose of photosynthesis. These cells are called mesophyll cells and they are supplied with water and nutrients through a network of intricate veins. These veins also carry carbohydrates produced by photosynthesis back to other areas of the plant. Leaves are covered with pores that will open up to take in carbon dioxide and close to prevent the loss of water. Evaporation of water from the leaf surface is stopped by a waxy coating called the cuticle.
The epidermis is under the cuticle layer, and is a single barrier layer of cells. The mesophyll layer of cells is next; these cells conduct most of the photosynthetic process. There are two types of mesophyll cells: palisade and spongy. Palisade cells process the sunlight needed for photosynthesis. Foliage on plants that are sun-loving has many palisade layers of mesophyll. Leaves of plants that do well in shade can have just one layer. Spongy mesophyll allows diffusion of oxygen, water vapor, and carbon dioxide through open stomata. Veins running through the leaf carry xylem and phloem as in stems. The next layer is the lower epidermis, which houses stoma that open and close during the process of transpiration.
Quick Question: What is the difference between 'foliage' and 'leaves'?
For the most part, these two terms are used somewhat interchangeably when talking about the appearance of a plant. For example, you can refer to multicolored vegetation as either 'variegated leaves' or 'variegated foliage'. However, technically, the word foliage is used to describe the collection of leaves on a plant, and the word leaves is used to describe the individual leaves themselves.
Parts of a Leaf
In addition to the interior parts of a leaf that aid in photosynthesis and the transport of water and nutrients, there are exterior qualities of a leaf that assist in identifying leaves and matching them to the plant they belong to. The apex refers to the tip of the leaf, and the margin refers to the edge. Veins act as structure and support for the leaf, as well as avenues to ​deliver water and nutrients. The midrib runs down the center of the leaf. The base refers to the bottom of the leaf where it attaches to the petiole, or the leafstalk, which connects the leaf to the stem of the plant. The stipule is a leaf-like appendage smaller than the leaf itself that appears at the spot where the petiole attaches to the stem of the plant.
Quick Question: Why are leaves green?
Leaves contain high amounts of chlorophyll, the pigment that harvests light energy for use in photosynthesis. It is stored inside chloroplasts. Chlorophyll absorbs all colors except for green, so when sunlight hits a leaf, green is the only color that is able to bounce off or pass through, making green the only color available for humans to perceive.
Types of Leaves and Venation
There are many types of leaves found on plants. For example, ferns have fronds, conifer-type plants have scale leaves, flowering plants have angiosperm leaves, and grasses have sheath-type leaves. There are also special leaf types that are modified for certain purposes, like seed leaves (also called cotyledons), spines, tendrils, bracts, and storage leaves. All of these leaf types are listed below, with descriptions and examples.
Frond Leaves
These types of leaves are found on all types of fern plants. These leaves are special in that they produce spores, which is how ferns reproduce.
Scale Leaves
These leaves are flat and scale-like, and are found on many types of conifer plants. Arborvitae plants have scale leaves.
Angiosperm Leaves
​These leaves are the 'normal' leaves people think of when they think of a leaf, and are comprised of a base, stipules, petiole and a blade.
Sheath Leaves
Often found on grasses, these leaves tend to be longer in comparison to the length of the stem than other leaf types, and are usually flat.
Seed Leaves (Cotyledons)
These leaves are found on embryonic plants and seedlings. They store food for the developing plant.
Spines
These are modified leaves that serve to protect the plant.
Tendrils
Modified stems that help to support and attach a plant to a structure.
Storage Leaves
This type of leaf is found on succulents and bulbous plants. They are used to store food.
Bracts
These leaves are often brightly colored. For example, the showy structures on Poinsettias are bracts, not petals.
The vascular bundles in a leaf extend from the stem, through the petiole, and into the leaf blade as veins. The term venation refers to how these veins are distributed. There are two principal types of venation: parallel-veined and net-veined (or reticulated). In parallel-veined leaves, numerous veins run essentially parallel to each other and are connected laterally by minute, straight veinlets. Parallel-veined leaves occur most often on monocotyledonous plants. The most common type of parallel veining is found in plants of the grass family, whose veins run from the leaf's base to its apex. In net-veined (or reticulated) leaves, veins branch from the main rib or ribs and subdivide into finer veinlets. These veinlets then unite in a complicated network. Net-veined leaves occur on dicotyledonous plants.

​In net-veined (reticulated) leaves, there are different names for the levels of venation. The central vein is called the midvein, and is a continuation of the petiole up through the leaf to the apex. Veins that directly branch off of the central vein are called secondary veins. ​Net venation may be either pinnate or palmate. In pinnate venation, the secondary veins extend laterally from the midrib to the edge. In palmate venation, the principle veins extend upward and outward, like the ribs of a fan, from the base of the leaf blade.
Leaf Shape and Arrangement
Leaves can be either simple or compound. Simple leaves have one piece to them, though they may be lobed or serrated, and compound leaves have two or more leaflets per leaf.  There are many variations of compound leaves, outlined below.
Simple
e.g. Acer rubrum
Unipinnate (Paripinnate)
e.g. Tamarindus indica
Unifoliate
​e.g. Citrus hystrix
Bifoliate
e.g. Hymenaea courbaril
Trifoliate
e.g. Trifolium
Quadrifoliate
e.g. Marsilea crenata
5-Palmate
e.g. Aesculus pavia
Multifoliate
e.g. Cannabis
Leaf shape can also vary considerably from plant to plant, and can even vary within the foliage of the same plant. Plants have evolved differently-shaped leaves to best survive in their environments, reducing water loss, capitalizing on available light absorption, and resisting damage from wind and other weather variables.
Linear
e.g. Typha latifolia
Oblong
e.g. Eucryphia
Lanceolate
e.g. Chrysolepis chrysophylla
Oblanceolate
​e.g. Embothrium coccineum
Spathulate
​e.g. Arabidopsis thaliana
Elliptical
​e.g. Ficus
Ovate
​e.g. Hoheria glabrata
Peltate
​e.g. Trapaeolum majus
Cordate
​e.g. Ipomoea purpurea
Obcordate
​​e.g. Muhlenbeckia astonii
Obovate
​e.g. Magnolia macrophylla
Rhomboid
​e.g. Hedera colchica
Deltoid
​e.g. Melissa officinalis
Hastate
​e.g. Hedera
Sagittate
​e.g. Sagittaria saggitifolia
Pinnatifid
​e.g. Quercus
Bilobed
​e.g. Ginkgo biloba
Palmately Lobed
​e.g. Liquidambar styraciflua
Palmate Compound
​e.g. Ptelea trifoliata
Pinnate Compound
​e.g. Juglans regia
Leaf Margins
​There are eight common types of leaf margins, referring to how the outer edge of the leaf looks. These include entire, scalloped, serrated, double serrated, toothed, spined, pinnately lobed and palmately lobed. Entire leaves have smooth edges, and scalloped leaves have edges that are uniformly wavy. Serrated leaves have saw-like teeth all pointing to the apex (tip) of the leaf. Double serrated leaves are similar, but have larger primary serrations edged with smaller ones. Toothed leaves are similar in looks to serrated leaves, but the teeth point outward, rather than toward the apex of the leaf. Spined margins have points to them. Pinnately lobed leaves have edges that come in towards the midvein, and palmately lobed leaves are similar, but edges come in towards the base of the leaf.
Leaf Attachments
Leaves are attached to stems in a number of ways, and identifying which way a leaf is attached to its stem can assist you in identifying the plant you are studying. The point on the plant where a leaf attaches to the stem is called a node.
Connate Perfoliate
The bases of opposite leaves are fused around the stem.
e.g. Silphium perfoliatum
Perfoliate
​Leaves have bases that completely enclose the stem.
e.g. Bupleurum rotundifolium
Clasping
Leaf with a base partly or entirely surrounding the stem.
e.g. Rudbeckia amplexicaulis
Sessile
Leaves are directly attached to the stem, without a petiole.
e.g. Helianthus divaricatus
Petiolate
Leaves are attached to the stem with a petiole, or leaf stalk.
e.g. Hibiscus
Sheathing
A tubular leaf attachment to a stem. 
e.g. Phleum alpinum
Leaf Attachment Patterns
There are many ways that leaves can be arranged on a stem, but there are three that are most common: opposite, whorled, and alternate. Opposite leaves occur in pairs, and the leaves are positioned directly across from each other on a twig. Whorled leaves are the same as opposite leaves, except there are three or more leaves ringing the twig. Alternate leaves are staggered along opposite sides of a twig, and are not directly across from each other. Remember that when leaves are not present, which happens on some plants when the leaves drop in the fall and winter or when they are removed from a plant, a specimen can still be categorized as alternate, whorled, or opposite by the positions of the leaf scars and buds.
Opposite
Whorled
Alternate
Leaf Size
Along with endless variations in shape, arrangement, color, and margins, leaves also vary in size. They can be as small as 1 millimeter and as large as 82 feet across. The benefit of larger leaves is increased surface area for the purpose of photosynthesis. Many plants with larger leaves live in areas where sunlight is limited; they have evolved to expand their surface area in an attempt to capture more incoming sunlight. These larger leaves, however, transpire more water which requires more water intake for the plant overall, but also cools down the plant in tropical regions. In cool, alpine regions, leaves are smaller to minimize frost damage, and in arid climates they are smaller, as well, to reduce the amount of water lost through stomata.
A Leaf's Job
The basic role of the leaf is photosynthesis, which is the process by which sunlight energy is combined with carbon dioxide and water to produce sugar and oxygen. The structure of the leaf is tailored to this process. Leaves are generally flat to provide as much surface area as possible for absorption of sunlight. What is more, the leaf is supplied with a system of veins that links up with the veins in the roots and stems. These avenues supply the water for photosynthesis and other nutrients needed by the photosynthesizing cells, and they carry sugars formed via photosynthesis to other parts of the plant, where they feed the living cells of the roots and stems or are stored for future use. Photosynthesis will be covered in more detail in the next module (Botany II).

flowers

Flowers are also known as blooms or blossoms, and although they are not found on every plant, they are critically important to the survival of many species. Most flowering plants depend on animal or insect activity to transfer their pollen between flowers, so there is a good reason why they are so eye-catching. Some flowers can also morph into fruits, providing food for animals and humans alike.

​The term flower in its common use refers to four basic parts of a plant: the sepals (collectively called the calyx), petals (collectively called the corolla), stamens (collectively called the androecium) and the pistils (collectively called the gynoecium). These parts are attached to the tip of a specialized branch called the peduncle (go ahead, say it a few times! Peduncle, peduncle, peduncle). The tip of the peduncle is called a receptacle.
The calyx (collection of sepals) is frequently green and produces food from sunlight via photosynthesis, just as leaves do. It protects developing flower buds in many plants. The corolla, the frequently colored part of the plant made up of petals, is located just inside the calyx. Showy corollas can attract pollinators such as insects or birds. Plants that are wind-pollinated frequently have minute corollas or lack them entirely, because they do not need to attract insect or animal pollinators to enhance reproduction
​
​The androecium is just inside the corolla. The individual stamens, the male reproductive organs that make up the androecium, are composed of an anther (usually a swollen structure) and a filament (a long stalk supporting the anther). With magnification, one can see that the anther is actually made up of sacs. It is in these sacs that pollen grains are produced. Pollen grains are capable of producing sperm, or male sex cells.
In the center of the flower, at the top of the receptacle, is the gynoecium. This is the female part of the flower, and can consist of one carpel or multiple. Each individual pistil (female reproductive organ made up of one or more carpels) typically looks like a flower vase and has three recognizable parts: a stigma, which will receive pollen grains at the top of the vase; a style, the 'neck-like' portion of the vase which supports and typically elevates the stigma, and an ovary, the swollen part at the bottom that will either open and release seeds or turn into a fruit. Eggs, the female sex cells, are produced in ovules, which are contained inside the ovary.
​It is important to note that the above is a description of a typical flower. In many species, flower parts may be fused with others, extremely reduced in size or prominence, or completely absent. These plants have evolved flowers to better reproduce in their native environments, which is reflected in the composition of their blossom parts. 'Perfect' or 'imperfect' and 'complete' or 'incomplete' are terms botanists use to describe the types of variation in flower composition. Perfect flowers (also called bisexual flowers) possess male (stamen) and female (pistil) parts, while 'imperfect' flowers (also called unisexual flowers) lack one of these parts. Complete flowers have sepals, petals, stamens, and pistils, while incomplete flowers lack one of these parts. Flowers can be perfect and complete, perfect and incomplete, or imperfect and incomplete, but never imperfect and complete.
Perfect, complete flowers are very common among flowering plants; an example is an Apple flower.
An example of a perfect incomplete flower is the Anemone, or windflower, which is missing petals. The colored parts in the photo above are actually sepals.
Watermelon flowers are great examples of imperfect, incomplete flowers. They are unisexual, meaning male flowers lack female parts and female flowers lack male parts.
Flowers cannot be imperfect and complete, due to the fact that an imperfect flower cannot have both female and male parts.
Inflorescences
The inflorescence refers to the part of the plant that holds flowers. There are two major classes of infloresences: terminal, in which the inflorescence is at the end of the a shoot (also called a scape) and axillary, in which the inflorescence is found in the axil of a leaf or along the shoot. The Tulip pictured below is an example of a terminal inflorescence, and the Bleeding Heart plant is an example of an axillary inflorescence.
There are many different types of inflorescences, depending on the composition of the individual flower attachments to stems, or peduncles. Check out the list below to explore them:
Capitulum
A single-flower inflorescence in which the flowers are found on a flattened surface called a receptacle.
Raceme
​An unbranched, elongated shoot with lateral flowers that mature from the bottom upwards.
Catkin
A pendulous spike of reduced flowers adapted to wind pollination; they dangle down from branches and allow the pollen to be freely dispersed.
Panicle
A raceme in which the lateral branches are themselves branched.
Single Flower
An inflorescence composed of a single flower.
Spadix
A spike with small flowers crowded on a fleshy axis.
Spike
A raceme with unstalked flowers maturing from the bottom upwards.
Umbel
A flat-topped inflorescence in which the peduncles all originate from a single point, much like the struts of an umbrella.

Conclusion

You've reached the end of your first module. Great job sticking it out, the first botany modules are a bit information-heavy. We hope that you've learned a lot. Before you give your brain a rest, check out the Resources & References section below. We'd also like to take this opportunity to introduce you to the Key Terms resource created to accompany this course. This aggregate Key Terms page will be linked at the bottom of every module.

Resources & References

Tree of Life Explorer
Additional videos to assist in a deeper understanding of the many concepts we've covered in this module:
A video that goes into depth about bontanical nomenclature
An Introduction to Botany
A crash course in Taxonomy​
All about roots​.