254 BOTANY: PRINCIPLES AND PROBLEMS 



therefore, do not grow more than a few centimeters high and 

 have never succeeded in producing a strong and vigorous land 

 vegetation. 



Success of the Pteridophytes. — It is a different matter with the 

 Pteridophytes, however. The dominant generation here, as we 

 have seen, is the sporophyte; and this new plant type, at least in 

 all the forms which have survived to the present day, seems to 

 be particularly well adapted to terrestrial life. Here for the first 

 time we meet with true roots — large, vigorous, much-branched 

 structures, each terminating in a mass of roots hairs and well 

 suited for rapid absorption and strong anchorage. The leaf, 

 instead of being a small and thin plate of tissue, is large and 

 relatively thick. It has a well-developed mesophyll of thin- 

 walled cells and is provided with abundant air spaces, the whole 

 structure being covered by a stout epidermis to cut down evapora- 

 tion. The necessary passage of gases between the outer air and 

 the internal tissues of the leaf takes place through characteristic 

 pores or stomata. The stem reaches a structural complexity 

 nowhere exceeded among plants, the tissues for support and con- 

 duction being particularly well developed. The evolution of the 

 true root, the true leaf, the stoma, and the highly differentiated 

 stem made it possible for Pteridophytes to produce the vigorous 

 and abundant land vegetation which covered the earth in ancient 

 times; and from this group have come the seed plants, which 

 form the bulk of the terrestrial vegetation of today. 



There is evidently a wide step between the mosses on the one 

 side and the terns on the other. Transitional forms which must 

 have existed between these two have entirely disappeared, and 

 we can only guess what the plants were like which connected the 

 Bryophytes and the Pteridophytes, and what were the first steps 

 in the evolution of the well-developed land-inhabiting sporo- 

 phytes which are so conspicuous today. This successful invasion 

 of the dry land stands out as one of the most important and 

 dramatic events in the history of the plant kingdom. 



6. The Evolution of the Seed. — The last great progressive 

 movement which we shall consider is the comparatively recent one 

 which carried the process of reproduction to a still higher degree 

 of efficiency and resulted in the development of that most perfect 

 of reproductive structures, the seed. 



The production of seeds is the distinctive feature of the 



