LIFE IIISTOEY OF A FERN 1 9 



firmly to the soil, and also serve to take in water and dis- 

 solved mineral nutrients. 



12. The Prothallus. Before the germ-tube has greatly 

 enlarged, it becomes divided into two cells, and then, 

 by successive cell-divisions, into an increasing number. 

 Meanwhile chlorophyll bodies begin to appear, but never 

 in the rhizoids. The final product of these cell-divisions 

 and growth is a tiny, flat, green body, often (but not 

 always) heart-shaped, with a central portion, the cushion, 

 several cells thick, and a marginal part, the wings, of only 

 one cell in thickness. Because of its flatness this little 

 plant (for such it is) is called a thallus] and because it 

 precedes, in the order of reproduction, the new sporophyte, 

 it is called the prothallus (Fig. 20). It is usually possible 

 to divide the prothallus into right and left halves, similar 

 in shape and in other characters, and hence it is said to 

 possess bilateral symmetry. 



