The life cycle of the Bryophyta is distinc- 

 tive. The haploid gametophyte generation is 

 dominant, whereas the diploid sporophyte 

 is relatively small and totally dependent 

 upon the gametophyte (Fig. 12-12). 



The bryophytes, undoubtedly, were among 

 the first plants to colonize the land. They 

 show only the most primitive adaptation to 

 land conditions. Bryophyta do not have any 

 well-developed vascular or epidermal tissues. 

 Also the absorbing organs are relatively sim- 

 ple rhizoids, rather than complex and effi- 

 cient roots. 



Bryophytes were among the first plants to 

 develop a multicellular archegonium, which 

 protects and nurtures the zygote and embryo 

 sporophyte. However, the flagellated sperm 

 of the bryophytes, after being liberated from 

 the antheridia, must swim through water in 

 order to reach the eggs. 



In view of these factors, the biological suc- 

 cess of the bryophytes has not been very con- 

 spicuous. Such small primitive land plants 

 were not able to offer serious competition to 

 the better adapted, more highly vascularized 

 Tracheophyta, which began to appear upon 

 the evolutionary scene early in the Devonian 

 period. 



The Bryophyta seem to have arisen from 

 the green algae (Fig. 29-11). This is indi- 

 cated by the filamentous protonema stage 

 (Fig. 12-2) in the bryophytic life cycle; by 

 the flagellated free-swimming sperm; and by 

 all available paleontological evidence. The 

 evidence also indicates that the Bryophyta 

 represent a terminal evolutionary branch. In 

 other words, the Bryophyta did not give rise 

 to the Tracheophyta. Probably, on the other 

 hand, the Bryophyta and Tracheophyta 

 originated from the same early stock, before 

 the distinctive features of either group had 

 been evolved. 



The Tracheophyta (Vascular Plants). This 

 diversified phylum (p. 594) includes virtually 

 all plants commonly seen in the garden, field, 

 and forest. These plants, in fact, have come to 

 dominate most of the land areas of the earth. 

 Today this phylum consists of more than a 



The Planf Kingdom - 611 



quarter of a million known species. More- 

 over, the fossil record, which extends back 

 more than 300 million years into the Devo- 

 nian and even the Silurian periods (Fig. 29- 

 10), reveals many species that later became 

 extinct. 



All tracheophytes, as the name implies, 

 possess vascular tissues, both xylem and 

 phloem (p. 244). This development has en- 

 abled many species to reach the dimensions 

 of imposing trees. Also all tracheophytes 

 show a regular alternation of diploid and 

 haploid generations. Moreover, the tracheo- 

 phytes have a distinctive life cycle. The 

 sporophyte generation is relatively large and 

 dominant, whereas the gametophyte is rela- 

 tively small and inconspicuous (see p. 219). 

 In fact, the gametophyte generation, among 

 higher Tracheophyta, is reduced to a micro- 

 scopically small thallus, which is entirely de- 

 pendent upon the tissues of the dominant 

 sporophyte for both sustenance and protec- 

 tion (p. 219). Thus when we speak of an 

 individual tracheophyte, such as a fern, a 

 pine tree, or a rosebush, we are referring to 

 the plant during the dominant sporophyte 

 stage of its life cycle. 



The Phylum Tracheophyta displays an 

 evolutionary divergence into four subphyla: 

 Subphylum 1. The Psilopsida: a virtu- 

 ally extinct group. 

 Subphylum 2. The Lycopsida: domi- 

 nant in Carboniferous era. 

 Subphylum 3. The Sphenopsida: al- 

 most extinct. 

 Subphylum 4. The Pteropsida: domi- 

 nant today. 



The Psilopsida (Subphylum h Phylum Tracheo- 

 phyta). Surviving today there are only three 

 species of Psilopsida, including the one 

 (Psilotum) that is depicted in Figure 31-19. 

 Fossilized Psilopsida, however, extend back 

 into the Silurian period, at which time they 

 showed a world-wide distribution. Some of 

 these fossils are so remarkably well preserved 

 that even the microscopic structure can be 

 studied. 



The Psilopsida, apparently, began to colo- 



