136 MANUAL OP BOTANY 



no differentiation into members. It is known as the pro- 

 thallium, and is usually a green flattened expansion of small 

 size. Its cells contain chloroplastids. From the under side 

 a number of root hairs are developed which attach it to the 

 soil. 



Sometimes the prothallium has a central somewhat thickened 

 portion known as the cusldon. Sometimes it is almost fila- 

 mentous, recalling the protonema of a moss. 



It always becomes quite free from the spore. 



On the under side of the prothalliimi the sexual organs are 

 developed, the antheridia lying to the basal end and the 



Fig. 894. 



Fig. 894. Protballus (gametophyte) of Fern. After Kny. 



archegonia, which are formed later, towards the anterior or 

 apical region. In a few cases a prothallium develops only 

 one or other of the sexual organs, but generally both are to be 

 found in the relative positions described. 



When the spore germinates, the outer coat ruptures and the 

 inner one grows out into a green filament consisting of a single 

 row of cells [fig. 895, 1-4). The end cell of the row soon divides 

 longitudinally, and the plate-like prothallium becomes recog- 

 nisable. The growth is soon continued by the formation of an 

 apical cell (fig. 895, 5), which after a while is found to lie in a 

 sort of notch or depression in the anterior margin. 



