Introduction to Cell Structure

The kingdom Plantae encompasses water-dwelling red and green algae as well as terrestrial plants, which have evolved to support themselves outside of the aquatic environment of their ancestors. This SparkNote focuses on the terrestrial plants, which include bryophytes (mosses) as well as the more highly evolved vascular plants, called tracheophytes. As a consequence of their move onto land, terrestrial plants require structures that support their weight, prevent desiccation (drying out), aid in reproduction, and transport water, nutrients, and the products of photosynthesis throughout the parts of the plant. Bryophytes have not yet made the complete transition to land, and are thus still dependent upon a moist environment to assist in reproduction and nutrient transport. The more highly evolved tracheophytes, on the other hand, have developed internal systems of transport and support called vascular systems, which have allowed them to become fully terrestrial.

As explored in Common Plant Characteristics , most terrestrial plants (both bryophytes and tracheophytes) share some general structural and functional features. Plant bodies are divided into two regions, the underground root portion and the aerial shoot portion (including stem, leaves, flowers, and fruits). These different regions of the plant are dependent on each other, as each performs different essential functions. Land plants also share certain more specific adaptations that are essential to survival out of water. These include an impermeable waxy cuticle on the outer aerial surfaces, jacket cells around the reproductive organs, and stomata that allow gas exchange without risking excessive water loss. All Plants are also autotrophic, meaning that they produce their own food and do not use other organisms to supply organic nutrients the way animals do. Finally, the life cycle of plants follows a pattern called the alternation of generations, in which they fluctuate between haploid and diploid generations and sexual and asexual modes of reproduction.

Terrestrial plants, as noted above, are classified as bryophytes and tracheophytes. Bryophytes, such as mosses and liverworts, are still dependent on a moist environment for reproductive and nutritive functions even though they are technically "terrestrial." Bryophytes also have very little internal support, limiting the heights to which they can grow. As a phylum, Bryophytes, are lower on the evolutionary scale than tracheophytes, which have adapted completely to life on land.

Tracheophytes (also known as vascular plants) possess well-developed vascular systems, which are comprised of tissues that form internal passageways through which water and dissolved nutrients can traverse the entire plant. Vascular plants are thus far less reliant on moist environments for survival. At the same time, Vascular systems also provide a strong system of support to the plant, allowing some tracheophytes to grow to immense heights. The vascular system will be discussed in far more detail in Vascular Tissues.

The tracheophytes can be further broken down into two kinds of seed-producing plants, gymnosperms (conifers) and angiosperms (flowering plants). The male gametes of gymnosperms and angiosperms are carried by pollen; each of these types of plants also produce seeds, which protect the embryos inside from drying out in a terrestrial environment. Angiosperms, with their flowers and fruits, have adapted even further to the terrestrial environment: flowers, by attracting insects and other pollen-bearing animals, aid in the transfer of pollen to female reproductive organs. Angiosperm fruits, developed from ovaries, protect the seeds and help in their dispersal. Finally, angiosperms themselves are divided into two classes--monocots and dicots--based on differences in embryonic development, root structure, flower petal arrangement, and other factors.

The seed, which develops from an ovule after fertilization has occurred, surrounds the plant embryo and protects it from desiccation. Each seed consists of an embryo, food source, and protective outer coat, and can lie dormant for some time before germinating. The roots of a plant function in the storage of nutrients, the acquisition of water and minerals (from the soil), and the anchoring of the plant to the substrate. Tiny root hairs, which extend from the root surface, provide the plant with a huge total absorptive surface and are responsible for most of the plant's water and mineral intake. Plant stems (or trunks, as they are called in trees) function primarily in nutrient transport and physical support. The leaves contain chlorophyll and are the major sites of photosynthesis and gas exchange. Flowers contain the reproductive organs of angiosperms.

Plants carry out a number of processes that are essential to their survival. Internal water and sugar transport are largely carried out within the vascular system, ensuring that the entire plant receives water and food even though these materials are brought in or produced only in certain parts of the plant. Plant hormones determine the timing and occurrence of many of the processes of the plant, from germination to tissue growth to reproduction. Plants can also respond to light, touch, and gravity in various ways.

The life cycle of plants depends upon the alternation of generations, the fluctuation between the diploid (sporophyte) and haploid (gametophyte) life stages. Reproduction in most plants can occur both sexually and asexually. In sexual reproduction, fertilization occurs when a male gamete (sperm cell) joins with an egg cell to produce a zygote. In gymnosperms and angiosperms (the seed plants), the ovule containing the egg cell becomes a seed after fertilization has occurred. In angiosperms (flowering plants), the embryo is given added protection by an ovary, which develops into a fruit. Plants can also reproduce asexually through vegetative propagation, a process in which plants produce genetically identical offshoots (clones) of themselves, which then develop into independent plants. This asexual means of reproduction can occur naturally through specialized structures such as tubers, runners, and bulbs or artificially through grafting.