1) A ) What critical adaptations arise in lineages of seedless vascular plants that promote increased size? B) Why do the gametophytes of seedless vascular plants stay small and close to the ground, while the sporophytes get larger and taller? Solution a) The vascular plants or tracheohphytes are the dominant and most conspicuous group pf land plants. More than 260,000 species or tracheophytes represent more than 90% os earth\'s vegetation. Byrophytes may have beebln successful at the transition from an aquatic habitat to land but they are still dependent on water for reproduction and absorb moisture and nutrients through the game to gametophyte surface. The lack of roots for absorbing water and minerals from the soil as well as lack of reinforced conducting cells limits the bryophytes to small sizes. Although they may survive in dry conditions they cannot reproduce and expand their habitat range in the absence of water. b) In seedless vascular plants the diploid sporophyte is the dominant phase of the life cycle. The gametophytes is now an inconspicuous but still independent organism. Throughout the plant evolution there is an evident reversal of roles in the dominant phase of life cycle. Seedless vascular plants still depend on water during fertilisation as the spermicide must swim on a layer of moisture to reach the egg. This step in reproduction explains why ferns and their relatives are more abundant in damp environments. By the late Devon Ian period plants had evolved vascular tissue, a well defined leaves and root systems. With these advantages plants increased in height and size..