550 LIFE-HISTORIES 



or increased in size or otherwise modified to provide for the 

 welfare of offspring. 



But however well these plants might succeed in utilizing under- 

 ground moisture they could never take fullest advantage of the 

 opportunities offered for life upon the land so long as they were 

 dependent upon surface water to secure fertilization; and every 

 fernwort still retains traces of its aquatic ancestry in the male 

 gametes which must swim to accomplish their purpose. Thus fern- 

 worts like mossworts are truly land-plants only during part of their 

 life, although the former have attained a prodigious development 

 upon land. 



Pteridophyta agree with Bryophyta in having archegonia, but are 

 distinguished among cryptogams by developing true roots, stems, and 

 leaves, in which a vascular system is developed. 



196. Cryptogams and phenogams. The highest develop- 

 ment of plant life is associated with the production of seeds, 

 which afford the best possible provision for the welfare of 

 offspring. 



There is abundant evidence to show that the earliest seed-plants 

 differed but little from certain fernworts that had developed ma- 

 crosporangia containing single macrospores. The first step toward 

 converting such a macrosporangium into a seed might easily be 

 taken if the macrospores remained attached to the plant until the 

 archegonia were exposed, while the microspores, set free, were car- 

 ried by wind to the attached gametophj'te there to germinate and 

 effect fertilization. The final step would come, when, after fertiliza- 

 tion of an egg-cell thus doubly protected by nurse-plant and spore- 

 sac, the nurse-plant itself should be further protected bj' continued 

 growth of the surrounding parts and should be fed by the parent 

 while it was in turn feeding the embryo. An embryo thus fed 

 through a connection maintained with the parent plant, and pro- 

 tected by a sporangium wall which finally becomes detached from 

 the parent for dispersal, is a seed; the macrosporangium with its 

 inclosed macrospore and female gametophyte is an ovule: the micro- 

 sporangia are anthers; and the microspores, pollen grains. When 

 such highly differentiated spore-sacs are borne upon leaves we have 

 sac-leaves which wc call either carpels or ■'itnmens. 



Pines and other gymnospermous plants (Figs. 258, 259, 260, 263) 

 as we have seen, bear ament-like clusters of stamens or carpels 

 each cluster forming what we may regard as a separate flower. A 

 Selaginella which had certain cones producing microspores exclu- 

 sively would thus be homologous with a staminate flower of Pinus, 

 while an exclusively macrosporic cone would correspond to a pis- 

 tillate flower. The morphology of the stamens and their parts in 



