First, the seeds of the angiosperms, a more recently evolved and highly successful division of the tracheophytes, are enclosed within a protective chamber, the ovary. A ripened ovary containing seeds is called a fruit. The fruit not only encloses the seeds but may also aid in seed dispersal. The fruit is either carried or eaten by animals that migrate to distant sites, carrying the seeds with them; the seeds are ultimately dropped or eliminated from the animals’ digestive tracts.
The seed also forms a little differently in angiosperms. One sperm nucleus from the pollen tube unites with an egg nucleus to produce the zygote. A second sperm nucleus unites with two haploid nuclei in the gametophyte (embryo sac) to produce the triploid endosperm, an important source of food within the seed for such seeds as corn.
In gymnosperms, pollination (transfer of pollen to female reproductive structure) can be only wind-borne. In flowering plants, pollen may be transferred by wind or by animals. Although insects have been stressed as prime agents for pollination, recent studies suggest that mice and other small mammals may also play a role, particularly in tropical plants.
A major internal modification of the angiosperms is the development of specialized xylem cells, the vessels and fibers, in addition to tracheids. The vessels are particularly significant because they are large-bore columnar cells that anastomose (join) end to end. When their inner cellular contents degenerate, they collectively form long tubes that greatly facilitate the passage of water in the plant. Fibers, on the other hand, function solely to provide support. In conifers, the single xylem elements, the tracheids, represent a more primitive condition