206 



PTER1DOPHYTA. 



the prothallium is generally heart-shaped, dark green, and pro- 

 vided with root-hairs, and it attains a 

 diameter of about one centimetre (Fig. 

 205). It is formed of one layer of cells, 

 except along the central line near the 

 anterior depression, where it becomes 

 several layers of cells in thickness, 

 forming the " cushion," on the lower side 

 of which the archegonia are developed. 

 The antheridia are first formed ; they 

 are thus found on the oldest parts of the 

 prothallium, on its edge, or among the 

 root-hairs. The archegonia are developed 

 later, and are therefore found near the 

 apex. Several tropical Ferns have pro- 

 thallia 1 deviating from this typical form; 

 Trichomanes (Order HymenopJiyllacece) 

 has filamentous, branched prothallia, 

 which resemble the protonema of a Moss. 



Others, again, have strap-shaped prothallia, which resemble the 



thallus of certain Liverworts. 



w' 



FIG. 205. Prothallium (pp) of 

 Maiden hair (Adiantum capillus 

 veneris) with a young plant 

 attached : b first leaf ; w' primary 

 root ; w" adventitious roots ; h h 

 root-hairs of the prothallium 

 ( x abt. 30). 



FIG. 203. Antheridia of Maiden-hair (x 550). A Unripe; B ripe, but unopened ; Copen 

 and ejecting the spermatozoids (s). Those which have been last ejected are still lying en- 

 closed in their mother-cells, the others are coiled up and drag with them the cytoplasmic 

 remains (b) ; /cells of the prothallium. 



1 In the Polypodiaceas unisexual prothallia as distinct as those of Equisetum 

 are of common occurrence. 



