416 PLANT MORPHOLOGY 



Bryophytes. The contributions of the bryophytes to the evolution of 

 the plant kingdom include the establishment of the land habit, the appear- 

 ance of archegonia and multicellular antheridia, and the development of a 

 distinct alternation of unlike generations as a constant feature of the 

 group. Prominent evolutionary trends include a differentiation of the 

 gametophyte in internal structure, the development of a leafy gameto- 

 phyte from a thallus, and a tendency of the sporophyte to become par- 

 tially independent of the gametophyte. From a sporophyte almost 

 wholly sporogenous, progress has been made by ever-increasing steriliza- 

 tion of tissue and its diversion to other functions. This is seen in the 

 development of a foot and seta, elaters, and a dehiscence mechanism, in 

 the formation of a columella, and in the development of green tissue. 



Relationships between the bryophytes and green algae are mainly con- 

 jectural, there being no direct fossil connection between the two groups. 

 However, the bryophytes seem clearly to have been derived from aquatic 

 ancestors, their structural advances being correlated with the establish- 

 ment of the land habit. These include a compact plant body, absorptive 

 rhizoids, jacketed sex organs, heavy-walled aerial spores, etc. 



Pteridophytes. The advance of the pteridophytes over the lower 

 groups is shown by the establishment of an independent sporophyte, 

 evolution of a vascular system, organization of a strobilus, and appear- 

 ance of heterospory. With few exceptions, the sporophyte consists of 

 roots, stem, and leaves. In one line of descent the leaves have remained 

 small, undivided, and single- veined ; in the other line they have become 

 large, divided, and many- veined. In the evolution of the vascular system 

 the trend has been from exarch to mesarch xylem and then to endarch, 

 also from a protostelic to a siphonostelic condition and then to a dictyo- 

 stelic one. The presence of leaf gaps in the ferns is regarded as an 

 advanced feature, their absence in other pteridophytes being primitive. 



Lycopods show an advance from those with every leaf a sporophyll to 

 those with a compact strobilus. Both lycopods and ferns show a trend 

 from homospory to heterospory. Heterospory has accelerated the reduc- 

 tion of the gametophyte generation and sex organs. Among homospo- 

 rous forms, subterranean gametophytes have been derived from aerial 

 ones. Ferns show evolutionary tendencies affecting the sporangia, such 

 as a change from the eusporangiate to the leptosporangiate type, reduc- 

 tion in the thickness of the wall and in the output of spores, development 

 of an annulus, etc. 



The origin of pteridophytes is uncertain, a direct connection with either 

 the algae or the bryophytes being hypothetical. Some paleontological 

 evidence indicates that the major groups have been derived independently 

 from the Psilophytales. 



