Plant Anatomy

Plant anatomy is the study of the shape, structure, and size of plants. As a part of botany (the study of plants), plant anatomy focuses on the structural or body parts and systems that make up a plant. A typical plant body consists of three major vegetative organs: the root, the stem, and the leaf, as well as a set of reproductive parts that include flowers, fruits, and seeds.

As a living thing, all of a plant's parts are made up of cells. Although plant cells have a flexible membrane like animal cells, a plant cell also has a strong wall made of cellulose that gives it a rigid shape. Unlike animal cells, plant cells also have chloroplasts that capture the Sun's light energy and convert it into food for itself. Like any complex living thing, a plant organizes a group of specialized cells into what are called tissues that perform a specific function. For example, plants therefore have epidermal tissue that forms a protective layer on its surface. They also have parenchyma tissue usually used to store energy. The "veins" or pipeline of a plant are made up of vascular tissue that distribute water, minerals, and nutrients throughout the plant. Combined tissues form organs that play an even more complex role.

THE ROOTS

A plant's roots, like the foundation of a skyscraper, help it to stay upright. They also absorb water and dissolved minerals from the ground and give the plant what it needs to make its own food. Most roots grow underground and move downward because of the influence of gravity, although the roots of some water plants float. Other root systems, like that of the English ivy, actually attach themselves to a vertical surface and allow the plant to climb. There are two main types of root systems: taproot and fibrous. Plants that have taproots grow a single, long root that penetrates straight down and firmly anchors the plant. Trees and dandelions have taproots that serve this function. Fibrous roots are shorter and more shallow and form a branching network. Grass has a fibrous root system that grows at a shallow level and in all directions. Inside a root are pipelines or veins that carry water and minerals to the rest of the plant. These pipes are concentrated in the center of the root, like the lead in the center of a pencil. At the end of each root is a cap that protects it as it pushes farther into the soil. Extending from the sides of the root, but further back from the root cap are root hairs. These hairs are the main water and oxygen absorbing parts of a plant. Materials enter and leave roots by two main processes: diffusion and osmosis. When molecules are distributed unequally, nature always seeks a balance and molecules will move from an area of high concentration to one of low concentration. When the cells of a root hair have little oxygen and the soil around the root hair has a lot, oxygen will move from the soil to the root automatically without the plant having to expend any energy. Osmosis is a similar situation (from high to low concentration), but it occurs when molecules, like those of water, move across a membrane that will not allow other materials to pass. Like diffusion, osmosis does not require the plant to use any energy.

THE STEMS

Plant stems perform two functions. They support the parts of the plant aboveground (usually the buds, leaves, and flowers), and they carry water and food from place to place within the plant itself. A stem is made up of an outer layer, the epidermis; an inner layer, the cortex; and a central zone called the pith. The stem of a green plant holds itself up by having thousands of cells lined up next to and on top of each other. As the cells take in water, they expand like a full balloon, and since their walls are elastic, they stretch very tight against each other and against the stem wall. It is their pressure that holds the stem up. A plant droops when its cells lack water and have begun to shrink. Woody plants, like trees, also contain a material called lignin that strengthen cell walls and make them more rigid. A plant's stem also functions as its circulatory system and uses what is called vascular tissue to form long tubes through which materials move from the roots to the leaves and from the leaves to the roots.

THE LEAVES

The leaf of a green plant manufactures food for plant growth and repair. A leaf is a highly specialized part of a plant since it is the place

STEPHEN HALES

English botanist (a person who studies plants) and physiologist (a person who studies how the many different processes going on inside a living thing actually work) Stephen Hales (1677–1761) is considered the founder of plant physiology. A pioneer in the study of blood circulation and blood pressure measurement, Hales applied the physics of his time to the problems of biology. In all of his experiments on plants and animals, he regularly emphasized the need for careful measurement of data.

Hales was born in Kent, England, and little is known of his life before he entered Cambridge University in 1696. There he studied science and religion, and in 1703 he was ordained in the church as a deacon (a clergyman just below a priest). In 1709 he became a clergyman at Teddington where he would remain for the rest of his life. At this time it was not unusual for a clergyman to also be a man of science, and Hales was able to do both well. It was at Teddington that Hales began to use some of the broad scientific education he had received and, in the spirit of English physicist and mathematician Isaac Newton (1642–1727), he tried to take what he knew of physics (the study of matter and energy) and apply it to biology.

Thus, in 1719, Hales began his first experiments on plants. Before this, he had done quite a bit of experimenting on animals and had achieved the first blood pressure measurements using a glass tube device of his own design. He also investigated the reflex actions in a frog whose head he had cut off, but after a while, Hales became in his own words, "discouraged by the disagreeableness of anatomical dissections." He therefore switched to plants and carried over his blood-related experiments on animals to the study of the movement of sap in plants. Soon he was able to measure the force of a plant's sap flow just as he had measured blood pressure in animals. In his book, Vegetable Staticks, published in 1727, Hales described many of his discoveries concerning plant physiology. Hales detailed what he had learned concerning plant anatomy and what a plant does in order to survive and grow. He stated that plants take in part of the air and use it for food, that they need light for growth, and that they lose water mainly through their leaves. He showed that sap is under considerable pressure and that water flows in a plant in one direction only. He even calculated the actual velocity (its speed) of the sap and discovered that it differs according to the type of plant. As he did in his animal experiments, he investigated the role of water and air in an organism and explored all aspects of its growth.

Hales also had a very practical and even humanitarian side, and he was a pioneer in the field of public health. He used his knowledge of air and respiration to devise ventilators to remove "spent," or bad air (probably carbon dioxide), from closed spaces in hospitals, prisons, and merchant ships. He worked on ways to distill fresh water from seawater, and worked at water purification and food preservation. He even adapted a gauge from his plant experiments to measure the ocean depths. Besides all of the specific botanical knowledge and understanding he offered in his book on plant physiology, Hales' application of physics to biology and his emphasis on quantitative (measurable) experimentation provided an important model for those who were to follow.

where photosynthesis takes place. In photosynthesis, the chlorophyll (green pigment) in the leaf absorbs energy from the Sun, combines it with water and minerals from the soil and carbon dioxide from the air, and produces the plant's food. Everything about a leaf is designed to intercept or capture sunlight. For example, a leaf is a flat structure with a large surface area and consists of a thin, flat blade called the lamina. The lamina is attached to a stalk called the petiole. The petiole is the leaf's main supporting rib and often branches into a network of veins. Leaves with only one blade are called simple, and those with two or more blades are called compound. Compound leaves often look like several small leaves attached to the same stalk. Leaves also grow in patterns to assure that they do not shade each other, and some plants have alternate leaves while others have leaves opposite each other. Leaves can control the amount of water they lose by opening or closing tiny slits called stomata (singular, stoma).

FLOWERS AND SEEDS

The reproductive part of a seed-producing plant is called the flower. Flowers have male and female cells that produce a seed when they unite. The stamen is the male reproductive organ in a flower and contains the male cells (pollen) in its anther that grows at the tip of its long, narrow stalk. The pistil is the female reproductive organ and looks like a long-necked bottle. It has a round base containing the ovary, a slender tube or long neck called the style, and a flattened, sticky top called the stigma. Once a flower opens, its petals (which are a type of leaf) protect the sex organs and serve to help pollination (the transfer of pollen to the female parts) by attracting animals like bees and birds. When this happens, fertilization occurs and the ovaries become seeds.

Seeds have three main parts: the coat, the embryo, and the food storage tissue. The coat protects the embryo, which is the beginning of a plant and grows by using food stored in the seed. Most seeds are enclosed in fruit that can be dry like a ripe bean pod, or fleshy like an apple or a peach. Other plants, like fir trees, have naked or uncovered seeds that form on the upper side of the scales that make up a pine cone. All are designed to be scattered as far as possible from the parent plant to ensure the further survival of the species.

[See alsoBotany; Photosyntheis; Plant Hormones; Plant Pathology; Plant Reproduction; Plants ]