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