The vascular plants encompass several divisions of plants and are collectively known as tracheophytes. Tracheophytes are characterized primarily by the presence of a vascular system composed of two types of specialized tissue: xylem and phloem. Xylem conducts water and minerals upward from the roots of a plant, while phloem transports sugars and other nutrients from the leaves to the other parts of the plant. Both xylem and phloem are distributed throughout the plant. The vascular tissue also serves as a means for mechanical support in the plant, so some tracheophytes (such as trees) can grow quite tall.
Vascular plants with unprotected seeds (gymnosperms)
The vascular plants having naked seeds are known as gymnosperms. Their seeds are not enclosed in female tissues and are therefore said to be naked. There are four divisions of gymnosperms: Cycadophyta (Cycads), Ginkgophyta (Ginkgo), Gnetophyta (Gnetae), and Coniferophyta (Conifer).
Coniferophyta is the largest and most familiar division of the gymnosperms. These plants are cone-bearers and are therefore called conifers. The seeds are borne on the surface of the female cone scales. Members of this division include trees such as cedars, firs, spruces, pines, and giant redwoods. The leaves are generally needle-shaped and contain vascular tissue.
The full-grown conifer (for example, a pine tree) is the sporophyte generation of the plant. The sporophyte produces male and female cones on the same tree. These cones produce spores that undergo meiosis and produce the male and female gametophytes. Male gametophytes are the pollen grains, each consisting of four cells. The male gametophyte produces sperm cells in the pollen grains. The female gametophyte produces two or three egg cells that develop within protective structures called ovules.
In the spring, the male cone releases pollen, which is blown about by the wind. Some pollen gets trapped on the female cone where it germinates and forms a pollen tube that makes its way into the ovule. A sperm cell then fertilizes the egg. The zygote that is produced develops into an embryo within the ovule. In time, the embryo matures to a seed. Eventually, the seed falls from the cone and germinates, and the germinating embryo becomes a new pine tree.
The function of dispersal in gymnosperms is assumed by the seeds. Growth of the embryo depends on food supplied from the parent sporophyte, where food-rich tissue surrounds the embryo. The gametophyte generation is little more than a reproductive mechanism in the gymnosperms. Both male and female gametophytes are tiny and entirely dependent on the parent sporophyte.
Vascular plants with protected seeds (angiosperms)
The angiosperms are the most developed and most complex vascular plants. They are the flowering plants, of which more than a quarter of a million species have been identified. Almost all vegetables, flowers, fruits, cereals, grains, grasses, and hardwood trees are angiosperms, the dominant life-form on earth today.
Angiosperm means “seed vessel,” a reference to the female tissues that enclose the seed. The tissue is endosperm. During embryonic development, the endosperm serves as a source of nourishment. In many angiosperms, the endosperm develops into the fruit of the plant. Thus, the protected seed is often found within a fruit. The two most distinguishing features of angiosperms are the flower and the fruit.