UH. XII] 



THE FERNS 



497 



archegoniurn^ and one of them fertilizes the egg cell T as al* 

 ready described (page 306). In most cases, antheridia and 

 archegonia are not ripe at the same time on a prothallus,. 



"Thus insuring a cross-tertiiization between different prothallia 

 (page 287)~ The fertilized egg cell divides into four parts, 

 which in the formation of the embryo develop respectively^ 

 the first root, leaf, and stem, with an absorbing FQOT^ which 

 remains in the prothallus and absorbs all available nourish- 

 ment therefrom until withered. The young root establishes 

 connection with the ground, the leaf rises and begins to 

 form new food, and soon the young 



~ 31ant is~growing"thrif tily into the 



FIG. 352. Thefibro- 



iar frondose Fern. 



In this, as in other Ferns, the 

 tissues are very highly differentiated, 

 being well-nigh identical, structurally 

 and functionally, with those in the 

 highest plants. The fibro-vascular 

 bundles are usually of a concentric 

 type, in which the phloem surrounds 

 the xylem ; and often they have re- cross-section; x i. (From 



TIT . v j i LeMaout and Decaisne.) 



markably specialized shapes as seen 

 in cross-section (Fig. 352). Commonly the epidermis con- 

 tains chlorophyll, a feature associated with their life in the 

 shade. Often they bear many chaffy or hairy growths, a 

 form of trichomes (page 19) ; and small adventitious, even 

 aerial roots, are readily produced. 



The identity of the two generations in the Fern is clear,' 

 for the prothallus is a typical gametophyte and the frondose 

 plant the sporophyte. Here, however, is involved a specially 

 interesting feature in the evolution of the, alternation of 

 generations, viz. both generations spread green tissue to light 

 and have their own connection with the soil, being thus in- 

 dependent plants as concerns their nutrition. The Ferns 

 are the only group in which this is true, for below them the 

 sporophyte is dependent more or less upon the gametophyte, 



2K 



