Nonvascular Plants—Bryophytes

    Bryophyta is the only phylum in the group of nonvascular seedless plants. These mosses and worts are the most primitive true plants. Because they lack a vascular system (vascular systems are discussed in much more detail in the section on Structure and Function of Plants, which is part of the Organismal Biology chapter), bryophytes do not have a stem, leaves, or roots; they must distribute water and nutrients throughout their bodies by absorption and diffusion. As a result, they cannot grow beyond a small size and must keep their bodies close to moist earth. Bryophytes reproduce by spores and need water in order to bring about fertilization. Because the male gamete is a flagellated sperm, reproduction requires water in which the sperm can swim. Unlike all other plants, which have a diploid adult stage, adult bryophytes are haploid, passing only briefly through a diploid phase during the reproductive cycle.

 

    Seedless Vascular Plants

    There are three phyla of seedless vascular plants: Lycophyta (club mosses), Sphenophyta (horsetails), and, most likely to appear on the SAT II Biology, Pterophyta (ferns). Vascular plants have a dual fluid transport system: xylem transports water and inorganic minerals from the roots upward, and phloem transports sugars and other organic nutrients up and down. This vascular system represents a major evolutionary step in the adaptation to life on land. The ability to transport water and nutrients across long distances allows plants to grow much larger, sending specialized photosynthetic structures (leaves) upward toward sunlight and specialized root structures downward toward the water and minerals in the ground. Like bryophytes, seedless vascular phyla reproduce by spores and have flagellated sperm that require water in which to swim, limiting these plants to relatively moist environments.

 

    Flowerless Seed Plants—Gymnosperms

    The evolution of seeds provided plants with another advantage in their prolonged pilgrimage onto land. Unlike the spores of more primitive plants, seeds are multicellular, containing both a complete diploid embryo and a food supply. Having a food supply inside the seed provides the newborn plant with a period of growth that is independent of food resources in the environment. This independence allows seed plants to grow in a greater variety of environments. Further freeing seed plants, the male gametes of the seed plants take the form of pollen, making reproduction independent of water.

 

    The seed plants that evolved first, called gymnosperms (“naked seeds”), do not produce flowers. Their seeds are exposed directly to the air, without any capsule or fruit enclosing them. The most important group of gymnosperms is phylum Coniferophyta; these plants, commonly called conifers, produce cones that carry seeds on their scales. Examples of gymnosperms are pines, firs, cedars, and sequoias.