Plant stems contain chlorophyll, but they are unable to produce food for themselves. Photosynthesis is mostly conducted by leaves, and the food produced is distributed to other sections of the plant. The stem's primary tasks are conduction and transportation. It provides support for the tree or herb and allows water and nutrients to be transported to the leaves where they are needed most.
Stems are usually divided into three parts: a root system at the bottom that attaches them to the earth; a trunk with branches at the top; and a middle section called a stalk that connects the root system to the head of the plant. The stalk may be hollow or solid depending on the type of plant.
In trees, the main stem is called the leader and it often grows straight up from the ground until it reaches a certain height when it begins to branch out. The leader sends its shoots upward in all directions to form more leaders which will continue to grow until they also send up new leaders. This process continues indefinitely, forming a large, spreading crown. At the end of each branch or twig is located a small leaf known as a scale. These leaves usually have three pairs of veins running vertically down the center, giving them a triangular shape. Beneath many species of plants, including oaks, ashes, sycamores, and cottonwoods, you can find a thick layer of brownish-red fibers known as bastes.
The major function of the stem is not photosynthesis. It is a leaf function. A stem's key roles are to transport water and nutrients from the roots to other parts of the plant, to generate branches, to carry flowers and fruits, and to keep the plant upright. The green color of stems is due to chlorophyll.
Stems also protect the rest of the plant by providing support for the weight of plants with large numbers of leaves or flowers, as well as for smaller plants that would be unable to support their own weight. They also help prevent damage to the plant when it is grown in crowded conditions. For example, branches growing toward the center of the plant will often grow longer than those growing toward the outside so they can reach light levels that allow photosynthesis to take place. This is called lateral growth and is responsible for the bushy appearance of many plants.
Stems are usually divided into three categories based on function: structural, supportive, and reproductive. Structural stems such as those of trees provide necessary support for the rest of the plant. Supportive stems such as those of grasses contribute to the stability of the soil surface. Reproductive stems such as cornstalks produce seeds. Plants use different strategies to achieve these results. Trees utilize growth hormones to promote stem elongation. Grass plants use their root system to find enough space to expand. Flowers develop into fruits to ensure reproduction.
Their primary role is to sustain the plant by carrying leaves, blooms, and buds; in rare situations, stems also serve as food storage for the plant. The stem also aids in the movement of photosynthetic products (i.e., sugars) from the leaves to the remainder of the plant. Finally, the stem provides support for the flower bud, which grows at the end of its longest petiole. If the stem does not reach sufficient length to connect with the flower bud, then the bloom will be born flat on the ground.
In addition to sustaining the plant, the stem plays an important role in the reproduction of plants. The reproductive organs are located near the top of the plant in structures called panicles on cereals or spikes on grasses. These organs include flowers, fruits, and seeds. All plants undergo a process of sexual reproduction that results in the formation of new individuals. This means that every plant is made up of two different types of cells: female cells contain the genetic information for making seeds; male cells contain this information but only make pollen. During sexual reproduction, the pollen from one plant cell joins with the ovules of another plant cell to form new plants. This process is called "fertilization" and the offspring of this union are called hybrids or clones. Plants without stems are referred to as cryptogams because they reproduce by means of spores produced in sacs within their tissues. Cryptogams include mosses and liverworts.
The stem's principal duties are to support the leaves, to convey water and minerals to the leaves, where photosynthesis converts them into useable products, and to transfer these products from the leaves to other sections of the plant, including the roots. The stem also supports the fruit as it develops, providing support during its growth and helping to break it off at maturity. The number of stems produced by a plant varies depending on the species; some produce only one, others several hundred or more.
Stems are usually divided into two parts: a central core and a peripheral vascular system. The central core is composed of loose groups of cells surrounded by a single layer of polygonal cells. The core of woody plants consists of several layers of cells surrounding a central strand of cellulose fibers. The outer covering of stems is called the bark. It provides protection for the stem and contains many useful chemicals that are released when it is damaged or burned. Some types of plants, such as cottonwood and willow, have very thin bark that does not protect their stems.
Vascular systems transport food and liquid to all parts of plants, including flowers, seeds, and branches. They do so by communicating through small holes called "stomata" on the underside of leaf blades. When CO2 enters through these stomata, it reacts with water to form H2O + carbon dioxide.