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 Coleochete, 
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 
