xv NATURE OF THE ALTERNATION OF GENERATIONS 569 



terrestrial plant, the gametophyte becomes more and more sub- 

 ordinated, finally serving merely to develop the reproductive 

 organs and to nourish the young sporophyte until it can take 

 care of itself. 



While it must remain conjectural just how the first true 

 root arose, the most probable explanation is that it was a modi- 

 fication of part of the foot. The foot is from its first inception 

 peculiarly an absorbent organ, acting much as the haustorium of 

 a parasite would do, and taking from the gametophyte the water 

 and food necessary for the growth of the sporophyte. The 

 foot, like the true roots developed later in the history of the 

 sporophyte, is a very different organ from the delicate rhizoids 

 of the gametophyte, and much more efficient for supplying a 

 massive structure like the sporophyte with the water necessary 

 for its growth. Moreover, as soon as a true root was estab- 

 lished, provided with an apical meristem for prolonged growth, 

 it could keep pace with the increasing size of the sporophyte, 

 and by the subsequent development of similar secondary roots 

 of increasing size and complexity, a root S3^stem was established, 

 to whose further development there was no apparent limit. 



So soon .as the sporophyte was emancipated from its depend- 

 ence upon the gametophyte, a new plant-type, essentially ter- 

 restrial in its nature, was established. This was not a trans- 

 formed aquatic organism, like the gametophyte, but the elabora- 

 tion of a structure essentially adapted to an aerial existence from 

 the beginning. To the zygote of some Alga, a resting spore 

 developed to carry the plant over a period of drought, can be 

 traced, step by step, by growth and specialisation, the complex 

 sporophyte as it exists among the vascular plants. 



This view of the origin of the leafy sporophyte from the 

 zygote of some aquatic algal ancestor is the so-called Anti- 

 thetic theory of alteration of generations. It assumes that the 

 two generations are essentially distinct, the gametophyte rep- 

 resenting the primitive aquatic phase, the sporophyte the sec- 

 ondary terrestrial condition, arising from the germinating 

 zygote. The sporophyte in its earliest condition was simply a 

 spore-bearing structure for the multiplication of the gameto- 

 phyte ; later is gradually assumed the character of an independ- 

 ent plant, of essentially terrestrial habit. 



Opposed to this view is the theory of Homologous Alterna- 

 tion. This theory was first championed by Pringsheim (3), 



