110 EVOLUTION OF PLANTS 



into equal parts, and this is followed by two other 

 walls at right angles to the first, and the globular 

 "embryo," as it is now called, is composed of eight 

 nearly equal cells. Soon there are formed a series of 

 walls by which a single layer of peripheral cells is sepa- 

 rated from the central mass of tissue (Fig. 28, B), and 

 the cells of the latter, after several preliminary divisions, 

 separate, and each one divides into four equal parts or 

 spores. This division is preceded by a double division 

 of the cell-nucleus, and it is not until the four nuclei 

 are complete that the division-walls arise between 

 them, by which the sporogenous cell is divided into 

 the four tetrahedral spores. These are at first thin 

 walled (F), but later develop a thick membrane (G), 

 and the spore as it ripens becomes filled with oil and 

 other nutritive substances. The mature sporophyte in 

 Riccia is simply a globular capsule, completely filled with 

 a mass of thick-walled spores. No assimilative tissue 

 is developed by the sporophyte, and it is entirely de- 

 pendent for its subsistence upon the gametophyte. The 

 venter of the archegonium continues to grow with the 

 enclosed sporophyte, and forms a protective covering 

 about it, much as do the enveloping cells in Coleochsete, 

 although in the latter the protective cells are entirely 

 undeveloped before fertilization. 



The mass of spores remains enclosed within the 

 archegonium-venter (" calyptra ") until they are liber- 

 ated by its decay, as the older parts of the thallus die. 

 After a period of rest, these spores germinate if they 

 are supplied with the proper conditions of light, heat, 

 and moisture. The spores give rise, not to a sporo- 

 phyte, but to a gametophyte, and it is interesting to 



