Tracheophytes are distinguished from bryophytes by their highly developed vascular systems, which facilitate the transport of water and nutrients to all parts of the plant. This vascularization adaptation has allowed tracheophytes to become more fully terrestrial than bryophytes, which are still dependent upon moist environments for many reproductive and nutritive functions, as discussed in Bryophytes. Tracheophytes can be broken down into three classes: ferns, gymnosperms, and angiosperms. Ferns are the least evolved of the tracheophytes; they have vascular systems, and specialized leaf and root structures, but are still dependent on moist environments for reproduction. Gymnosperms (coniferous plants) and angiosperms (flowering plants), known together as the seed plants, have evolved reproductive processes that are independent of water. In addition, tracheophyte seed plants all produce embryos that are encased in tough coats. These seed coats prevent desiccation in a terrestrial environment and protect the seed until conditions are favorable for growth. Angiosperms can be further classified as monocots and dicots, depending on their embryonic development and other factors.
Tracheophytes are characterized by the presence of vascular tissue, composed of specialized conductive cells that create "tubes" through which materials can flow throughout the plant body. These vessels are continuous throughout the parts of the plant, allowing for the efficient and controlled distribution of water and nutrients. In addition to this transport function, vascular tissues also provide a measure of support to the plant, contributing to tracheophytes' ability to grow much larger and higher than nonvascularized plants. The two types of vascular tissue are xylem (dead cells) and phloem (living cells). Roots and root hairs, through which the bulk of water and minerals enter the plant body, are also integral to the vascular system of tracheophytes.
Xylem consists of a "pipeline" of dead cells arranged end to end for water and mineral transport. When the cells that form xylem die at maturity, the nucleus and cytoplasm disintegrate, leaving a hollow tunnel. The leftover cell walls are very thick and provide support for the plant; the cavities inside provide a passage through which fluids can move. The xylem carries water and dissolved minerals upward from the roots through the stem and leaves of the plant. In larger seed plants, xylem cells are specialized into vessel elements and tracheids. Vessel elements are found in flowering plants (angiosperms), and are wider and better at conducting water than the tracheids of conifers (gymnosperms).
Unlike xylem, the cells that make up phloem are living at maturity and can carry materials both up and down the plant body. Phloem is comprised of sieve elements, which are arranged end to end to form passageways, and companion cells, which are closely associated with the sieve elements even though their exact function is unknown. Mature companion cells have both a nucleus and cytoplasm, while the mature sieve elements contain only cytoplasm; for this reason it is thought that the nuclei of companion cells may control the activities of neighboring sieve elements. Phloem is responsible for distributing the products of photosynthesis, such as amino acids and carbohydrates, from the leaves to the rest of the plant.
Like the stem, the roots of a tracheophyte contain vascular bundles composed of xylem and phloem. Although the relative position of the two tissues is different, the transport system within the roots is continuous with that in the shoot, allowing for the efficient movement of water and nutrients up and down the plant body. The roots draw water and minerals from the soil and pass them upward to the stem and leaves. They are also responsible for storing the plant's organic nutrients, which are passed downward from the leaves through the phloem. Radiating from the roots are a system of root hairs, extend from the surface of the root and vastly increase the absorptive surface are of the roots. For more information on roots and root hairs, see Roots, Plant Strucutres
Vascular plants undergo two kinds of growth, called primary and secondary growth. Primary growth occurs in the apical meristems, located at the tip of both root and shoot, and is mainly a growth of vertical length. The meristems are regions of rapid mitotic division, almost a cell-making factory. When a cell divides, one of its offspring moves down into the plant body where it elongates, and the other remains in the meristem to divide again.
Secondary growth is a growth of thickness. Secondary growth is a product of two different, though related, tissues, which both fall under the umbrella term lateral meristems. Vascular cambium exists between xylem and phloem; on its inside the cambium produces what is known as secondary xylem, on its outside it forms secondary phloem. The primary xylem and phloem are pushed further inward and outward, respectively. As xylem gets older it often becomes clogged and ceases to function; this tissue is called heartwood, in direct contrast to sapwood, which comprises the functioning xylem cells. The vascular cambium is more productive during the growing seasons. During the rest of the year it creates little growth. This phenomenon creates distinct rings of growth, each ring representing a single growing season. By studying these rings, it is possible to calculate the age of a plant, and even possible to determine the specific conditions of a particular growing season. The second lateral meristem is called cork cambium, and is responsible for the formation of cork (bark), which replaces the epidermis is the protective covering of shoot and root.
Gymnosperms are commonly known as conifers, and includes spruces, firs, hemlocks, and othercommon evergreens. Like all tracheophytes, gymnosperms contain vascular tissues. They have developed seeds and nonflagellated sperm; male gametes (carried inside pollen grains) are moved by the wind instead of through water.
The dominant phase in the gymnosperm life cycle is the diploid (sporophyte) stage; the gametophytes are very small and cannot exist independent of the parent plant. Male and female cones, the reproductive structures of the sporophyte, produce two different kinds of haploid spores: microspores (male) and megaspores (female). This phenomenon is called heterospory. These spores give rise to gametophytes of the same sex, which in turn produce the gametes.
Gymnosperm are also characterized by a specialized fertilization process, involving differentiated male and female gametophytes. Fertilization occurs when pollen grains (male gametophytes) are carried by the wind to the open end of an ovule, which contains the eggs, or female gametophyte. For a (much) more detailed exploration of this process, see gymnosperm fertilization
Angiosperms are typically divided into two classes: monocots (including grasses, grains, and spring-flowering bulbs such as daffodils) and dicots (including oaks, elms, sunflowers, and roses). As discussed in Xylem and Phloem , angiosperms have a vascular advantage over gymnosperms. The vessel elements in their vascular tissue, which evolved from the tracheids found in conifers, are more specialized for conducting fluids. In addition, fibers within angiosperm xylem give added support to the plant structure. Another positive adaptation that is exclusive to angiosperms is the flower, which attracts insects and thus facilitates the transfer of pollen. Flowers, the reproductive structures of angiosperms, take the place of gymnosperm cones. Furthermore, while the ovules of gymnosperms are exposed on the surface of the cone), angiosperm seeds (which develop from ovules) are enclosed within an ovary. This ovary later matures into a fruit, which aids in the dispersal of the seeds (through animals or wind) and protects the seeds from drying out.
Much of the angiosperm life cycle resembles that of gymnosperms. The sporophyte stage dominates, and the gametophytes are even smaller than those of gymnosperms. The mature diploid plant produces male and female haploid spores through heterospory, which gives rise to single-sex gametophytes, which in term produce gametes. These gametes, through either self-pollination or cross-pollination, join to form a diploid zygote that eventually becomes a seed for a new angiosperm. For more information on the life cycles of angiosperms see Alternation of Generations.
Monocots, a class of angiosperms that includes grasses, grains, and spring- flowering bulbs, are named for the presence of a single cotyledon (leaf) during embryonic development. In general, the veins of monocot leaves are parallel, the flower parts occur in multiples of three, and a fibrous root system is present. Bundles of vascular tissue are scattered throughout the stem instead of appearing in a single ring.
Dicots, such as maples, oaks, elms, sunflowers, and roses, originate from embryos with two cotyledons. They are further distinguished from monocots by the network of veins in their leaves, the occurrence of their flower parts in groups of four or five, and the presence of a taproot. The vascular bundles of dicots are arranged in a tubular pattern in the stem.