114 



THE BOOK OF CHOICE FERNS. 



give it its colour. The prothallus varies in shape and in size according to 

 the genera, and also according to its age : in the majority of cases it is, when 

 only partly developed, much longer than broad (see Fig. 1) ; but it rapidly 

 extends in breadth until, when full grown, it generally resembles in outline 

 the conventional figure of a heart, having one end narrowed and a notch in 

 the broader margin (see Fig. 2), and seldom exceeds Jin. in breadth. Its 

 lower surface then becomes partly covered with tiny root-hairs, each made 

 up of a row of cells. It is also on its lower surface, and along the edges, 

 that the " antheridia," or male organs, are disposed. 



The "antheridia," which are usually numerous, and exist in the form 

 of small, cellular protuberances composed of three comparatively large cells, 

 originate as outgrowths of cells of the epidermis ; each outgrowth is cut off, 



Fig. 7. Young Prothallus 



(much magnified). 



p, Prothallus ; rh, Root-hairs of 

 Prothallus ; s, Spore. 



Fig. 2. Lower Surface of Mature 

 Prothallus 



(magnified). 

 a, Archegonia ; rh, Root-hairs. 



Fig. 3. Antherozoicl 

 (much magnified). 



as a new cell, by a cell-wall. In some prothalli, the contents of the cell thus 

 formed, and containing a tissue of mucilaginous nature, break up into a 

 number of small, rounded cells, called " parent cells," in each of which is 

 formed an "antherozoid" (see Fig. 3), slender, but coiled spirally in two 

 or three turns, and provided with a tuft of fine hairs or cilia at one end. 

 These antherozoids (from anther os, belonging to a flower, and zoon, life), 

 which somewhat resemble microscopic tadpoles, and swim actively in the 

 moisture surrounding them, are the analogues of the pollen- grains in flowering 

 plants, while the pollen -cases are represented, in Cryptogams, by the antheridia. 

 There is no doubt that, however different in other details, the physiological 

 action of the antherozoid is similar to that of the pollen -grain, as the only 

 function of this organ is fecundation ; this is effected by means of a more or 

 less rapid motion, to which it is subjected by the action of a number of 



