176 GENERAL BIOLOGY 



the liverwort thallus. This is the sporophyte. It 

 develops at the expense of the archegonial tissue and 

 matures within itself great numbers of minute 

 spores. When these sprout, they grow into a plant- 

 body resembling the original " liverwort," - that 

 is, the gametophyte. 



Much the same sort of development is to be 

 observed in the mosses. The sporophyte here grows 

 into a long stalk (drawing upon the tissues of the 

 gametophyte for its sustenance), at the end of which 

 is borne the sporangium. From each of the multi- 

 tude of tiny spores sprouts another gametophyte 

 or moss-plant, and so the cycle is completed. The 

 sporophyte of mosses, unlike that of liverworts, 

 although mainly dependent s upon the gametophyte 

 tissue for its food supply, is, nevertheless, provided 

 with some chlorophyll-tissue, and hence can manu- 

 facture some food of its own. 



Ferns. In the fern, the familiar leaflike plant 

 is the sporophyte ; the gametophyte is a tiny, heart- 

 shaped thallus, somewhat like a very simple liverwort. 

 The gametes of both sexes develop on the lower 

 side of the thallus (prothallium). The sperm are 

 motile and swim to the egg-cell in the dew or rain. 

 The fertilized egg divides into a mass of cells within 

 the antheridium, drawing its sustenance, as in the 

 mosses, from the tissue of the gametophyte. How- 

 ever, it soon begins to develop a green, leaflike 

 branch, that grows upward, and a root-stalk that 

 grows into the earth. Thereupon it is independent of 



