92 Plant Life and Evolution 



Each of the inner cells divides into the four spores, 

 a constant character in all archegoniates. These 

 spores are usually capable of resisting drought, and 

 correspond physiologically to the single zygote of 

 the algae. The retention of the embryo sporophyte 

 within the archegonium, and its nourishment at the 

 expense of the gametophyte, enable it to prolong its 

 period of growth, with a corresponding ability to 

 increase its output of spores a great advantage to 

 the plant, as a single fertilization thus results in a 

 very much increased number of spores as compared 

 with the simpler algae. 



Even in the simplest sporophyte, like that of 

 Riccia, a small amount of tissue remains sterile, i.e., 

 does not give rise to spores. In all the other types 

 the amount of sterile tissue is very much increased, 

 and it soon begins to develop into special structures, 

 indicative of a division of labor, and this involves 

 a much longer growing period for the developing 

 sporophyte. In most liverworts (Fig. 10, B) the 

 lower part of the embryo is very early separated 

 from the upper portion from which the spores are 

 developed, and this lower part becomes a definite 

 organ of absorption, the foot, by means of which 

 food is taken from the tissues of the gametophyte 

 for the nutrition of the embryo-sporophyte, which 

 thus may be said to live parasitically upon the 

 parent gametophyte. Other structures may also 

 develop from the sterile tissue, such as the 

 elongated seta or stalk, which may reach a con- 



