170 PLANT-LIFE 



they are an anticipation of the seed-bearing plants in 

 which the ovules are fertilized, and the embryo is formed 

 and supplied with some reserve food in the seed itself, 

 all being accomplished before the seeds are shed. The 

 microspores of the same plant correspond to the pollen 

 grains of the seed-plants, and the sporangia in which they 

 are formed to the pollen-sacs. 



Along with the greater sex-differentiation we observe 

 a corresponding reduction of the prothallus, which in 

 Selaginella has lost the power of independent nutrition 

 in the female, and in the male has but a single vegetative 

 cell. Herein we see a further determined effort towards 

 independence of the sporophyte. In the conquest of the 

 land, plant life has evidently advanced cannily. It has 

 felt its way, and even groped in the dark; but from 

 the evidence before us it certainly did not " burn its 

 boats behind it " until it could assuredly get on without 

 them. The gametophyte was first aquatic, then am- 

 phibious, and finally, as we shall see later, terrestrial, 

 and in each stage of its advance it became more reduced 

 vegetatively, and more specialized for sexual purposes. 

 In this advance the sporophyte increases mightily, and 

 in the end asserts and justifies its independence. 



We must also insist upon the importance of the 

 development of a vascular system in the Pteridophytes ; 

 it stands for adaptation to terrestrial conditions, and an 

 elaboration of structure to meet their requirements. If 

 a plant is determined to live an upright and partially 

 aerial existence, it must strengthen itself against the 

 wind, and, moreover, provide water and ventilating 

 systems that are equal to its needs. 



