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 ferilS, 



fern (Onoclea struthiopteris) , showing the rup- Vyrif if rna,V reach 

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



a somewhat older plant (gametophyte) with a a length of S6V- 

 single apical cell, x; C, female gametophyte 



seen from below, showing the archegonia, ar; eral Centimetres, 



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



gametophyte, g\ the sporophyte has developed Drancmng 



leaves and roots, r, so that it is quite inde- 4- QT , :, T i Tr 



pendent of the gametophyte. tensively, 



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 (Hymenophyllacese) 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 



6X- 



