Roots and Shoots
Terrestrial plants share a few defining characteristics, structural as well as functional. Perhaps the most basic shared feature of most plants is their division into shoots and roots. The separation between these two portions of the plant came about during the evolutionary move from an aqueous environment to a terrestrial one, and each part is essential in its own way to the plants' ability to survive on land.
The root, defined as the portion of a plant beneath the soil, brings in essential water and minerals from the soil. It also anchors the plant to the substrate, providing stability.
The shoot includes all aerial plant structures such as stems, leaves, flowers, and fruits. The shoot gathers the carbon dioxide and light energy necessary for photosynthesis, provides surfaces for gas exchange, and contains the plant's reproductive organs. Each of these parts, the root and the shoot, is dependent on the other, for roots cannot perform photosynthesis and shoots cannot take in water and inorganic nutrients.
Prevention of Water Loss
Plants share other structural qualities as well, most of which stem from their adaptation to terrestrial conditions. All plants have reproductive structures that prevent desiccation (drying out) of the gametes. These sex organs, called antheridia (male) and archegonia (female), are themselves covered by a layer of jacket cells that help to retain moisture.
In addition to the protection given to the sex organs, the plant surfaces exposed to air are covered in a waxy layer, called a cuticle, that guards against water loss. Gas exchange in plants is limited to pores in the leaf epidermis called stomata, which can open and close to prevent excessive evaporation of water into the environment.
Most plants are autotrophs, organisms that synthesize all their own organic nutrients and do not rely on other organisms for food. The reason that plants are autotrophic is that they carry out photosynthesis in their leaves. In the process of photosynthesis, the plant converts water, carbon dioxide, and light energy into oxygen, sugars, and more water. The oxygen is released into the surrounding air through the stomata, and the sugars (organic nutrients) are transported throughout the plant body to areas of growth and storage.
Alternation of Generations
Finally, plants undergo a life cycle that takes them through both haploid and diploid generations. The multicellular diploid plant structure is called the sporophyte, which produces spores through meiotic division. The multicellular haploid plant structure is called the gametophyte, which is formed from the spore and give rise to the haploid gametes. The fluctuation between these diploid and haploid stages that occurs in plants is called the alternation of generations. For further discussion, see Life Cycle, Alternation of Generations .