230 MOSSES AND FERNS CHAP. 



Many Bryophytes can exist only in damp, shady localities, 

 and those which have adapted themselves to a xerophytic habit, 

 have acquired the power of becoming completely dried up with- 

 out being killed, reviving promptly when supplied with water, 

 but remaining completely dormant during the period of 

 drought. These plants do not depend upon their rhizoids for 

 absorbing water, but, like Algae, can absorb water at all points 

 of their surface. Where the plant depends largely upon the 

 rhizoids for water absorption, as in the Marchantiacese, the 

 plant is a flat, prostrate thallus, which offers a large surface for 

 the development of the rhizoids. In the upright stems of the 

 larger Mosses, the rhizoids are multicellular, and sometimes 

 twisted into root-like strands, which are of relatively large size, 

 and are undoubtedly efficient organs for water-absorption. 

 Still it is evident that even such strands of multicellular rhizoids 

 would not suffice for providing the water necessary to make 

 good the loss by transpiration in a large terrestrial plant. 

 It is this failure to develop an adequate root system which prob- 

 ably explains the fact that no Bryophyte has attained the dignity 

 of a successful upright terrestrial plant. 



Among the Pteridophytes the gametophyte is equally in- 

 capable of a strictly terrestrial existence; but in these plants, 

 the sporophyte, developing still further along lines indicated in 

 many Bryophytes, has finally attained to the condition of an 

 independent plant. It may be conjectured that from part of 

 the foot, the absorbent organ of the embryo in the bryophytic 

 sporophyte, there was developed a root, with a permanent grow- 

 ing point, and capable of indefinite growth in length. This, 

 penetrating through the tissues of the gametophyte, put the 

 sporophyte into direct communication with the water in the 

 earth, and thus completely emancipated it from its former status 

 of dependence upon the gametophyte. 



The true root differs essentially from the rhizoids in being 

 a massive organ capable of indefinite growth and division, 

 which can thus keep pace in its development with the increasing 

 size and complexity of the sporophyte. The latter from this 

 time assumes more and more the principal role in the life- 

 history of the organism, while the gametophyte becomes corre- 

 spondingly reduced. With the development of an independent 

 sporophyte, there appeared a plant adapted from the first 

 to a terrestrial existence and not a modification of an originally 



