The Origin of Land Plants 91 



In those algae which are assumed to be the near- 

 est relatives of the archegoniates, the zygote on 

 germination produces by division of its contents 

 several spores which in most cases are motile zo- 

 ospores. This division of the zygote's contents into 

 several spores, each of which produces a new plant, 

 gives of course an advantage over those forms in 

 which the zygote develops at once into a single 

 plant. In one of these green algae, Coleochaete, 

 there is a material growth in the size of the egg 

 after it has been fertilized, and when the spore 

 germinates there is developed a comparatively large 

 multicellular body which resembles the embryo- 

 sporophyte formed in the liverwort, and is the near- 

 est approach to this structure that has yet been 

 discovered among the algae. Whether or not this 

 resemblance indicates a true relationship has been 

 much discussed and is still not satisfactorily settled. 

 It is pretty evident, however, that the sporophyte of 

 the first archegoniates must have been derived from 

 some structure which could not have been very dif- 

 ferent from the sporophyte of Coleochaete. 



A study of the evolution of the sporophyte in the 

 lower existing archegoniates demonstrates clearly 

 the course of development leading up to the higher 

 plant types. In Riccia, for example, the globular 

 mass of tissue derived from the growth and division 

 of the egg has practically all of its cells devoted 

 to spore formation, there being only a single layer 

 of sterile tissue upon the outside (Fig. 10, A). 



