126 



EVOLUTION OF PLANTS 



duces a slender filament or cell-row, much like the sim- 

 pler green algae. This condition soon gives place to a 

 delicate flat thallus, closely resembling some of the 



simpler liver- 

 worts. At this 

 stage growth is 

 effected by a 

 single apical cell 

 (Fig. 32, B, x) 

 precisely as in 

 such simple liv- 

 erworts as An- 

 eura. The degree 

 of development 

 of this thalloid 

 gametophyte 

 varies much in 



FIG. 32. A, the germinating spore of the ostrich different terilS, 

 fern (Onoclea struthiopteris) , showing the rup- Vvnf jf m^y reach 



tured spore-coat, sp, and the first rhizoid, r\ B, 



a somewhat older plant (gametophyte) with a a le 11 Of til of S6V- 



singie apical cell, x; C, female gametophyte 



seen from below, showing the archegouia, ~ar; eral centimetres, 



D, young sporophyte, sp, still attached to the -i -, . 



gametophyte, g ; the sporophyte has developed brandling 6X- 



leaves and roots, r, so that it is quite iiide- fonaT-tT-QK,- mirl 

 pendent of the gametophyte. ivei j? 



living for sev- 

 eral years, especially when the archegonia remain unfer- 

 tilized (Fig. 31, A). The largest of these "prothallia' 

 occur in certain tropical ferns, especially species of 

 filmy-ferns (HymenophyllacefB) and Vittaria. In the 

 latter genus they sometimes have numerous branches, 

 radiating from a common centre and forming circular 

 disks ten centimetres or more in diameter, and closely 

 resemble a large liverwort. These large gametophytes 



