TYPES OF CRYPTOGAMS; PTERIDOPHYTES 289 



cover with a large cover-glass, and examine with the lowest power 

 of the microscope. Note : 



(a) The abundant root-hairs, springing from the lower surface 

 of the prothallium. 



(&) The variable thickness of the prothallium, near the edge, 

 consisting of only one layer of cells. 



(c) (In some mature specimens) the young fern growing from 

 the prothallium, as shown in Fig. 211, B. 



The student can hardly make out for himself, without much 

 expenditure of time, the structure of the antheridia and the arche- 

 gonia (Fig. 211, 4), 

 by the cooperation 

 of which fertilization 

 takes place on much 

 the same plan as that 

 already described in 

 the case of mosses. 

 The fertilized egg- 

 cell of the archego- 

 nium gives rise to 

 the young fern, the 

 sporopJiyte which 

 grows at first at the 

 expense of the parent 

 prothallium but soon 

 develops roots of its 

 own and leads an in- 

 dependent existence. 



353. Nutrition. 

 The mature fern 

 makes its living, as flowering plants do, by absorption of nutritive 

 matter from the soil and from the air, and its abundant chlorophyll 

 makes it easy for the plant to decompose the supplies of carbon 

 dioxide which it takes in through its stoinata. 



FIG. 211. Two Prothallia of a Fern (Aspidium). 

 A, under surface of a young prothallium ; ar, arche- 

 gonia ; an, antheridia ; r, rhizoids ; B, an older pro- 

 thallium with a young fern-plant growing from it ; 

 I, leaf of young fern. (Both x about 8.) 



