126 EVOLUTION OF PLANTS 
duces a slender filament or cell-row, much like the sim- 
pler green alge. 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. 382, B, x) 
precisely as in 
such simple liv- 
erworts as An- 
eura. The degree 
& of development 
of this thalloid 
gametophyte 
varies much in 
Fic. 32, 7 A, pe germinating spore of the oe different ferns, 
fern (Onoclea struthiopteris), showing the rup- ; ¢ 
tured spore-coat, sp, and the ‘first rhizoid, 7; B, but it may: reach 
a somewhat older plant (gametophyte) with a a, length of sev- 
single apical cell, x; C, female gametophyte 
seen from below, showing the archegonia, ar; eral centimetres, 
D, young sporophyte, sp, still attached to the b hi 
gametophyte, g; the Bporotyte has Sor iped ranching exX- 
leaves and roots, r, so that it is quite inde- 2 
pendent of the gametophyte. tensively. ’ and 
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 (Hymenophyllacee) 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 
