PTERIDOPHYTES. 109 



the veins contain numerous chloroplasts, so that the little 

 plant is now quite independent of the prothallium, which has 

 hitherto supported it. As soon as the fern is firmly established, 

 the prothallium withers away. 



Comparing this now with the development of the sporo- 

 gonium in the bryophytes, it is evident that the young fern is 

 the equivalent of the sporogonium or spore fruit of the former, 

 being, like it, the direct product of the fertilized egg cell ; and 

 the prothallium represents the moss or liverwort, upon which 

 are borne the sexual organs. In the fern, however, the sporo- 

 gonium becomes entirely independent of the sexual plant, and 

 does not produce spores until it has reached a large size, living 

 many years. The sexual stage, on the other hand, is very much 

 reduced, as we have seen, being so small as to be ordinarily 

 completely overlooked ; but its resemblance to the lower liver- 

 worts, like Riccia, or the horned liverworts, is obvious. The 

 terms oophyte (egg-bearing plant) and sporophyte (spore-bear- 

 ing plant, or sporogonium) are sometimes used to distinguish 

 between the sexual plant and the spore-bearing one produced 

 from it. 



The common maiden-hair fern (Adiantum pedatum) has been 

 selected here for studying the structure of the full-grown 

 sporophyte, but almost any other common fern will answer. 

 The maiden-hair fern is common in rich* woods, and may be at 

 once recognized by the form of its leaves. These arise from a 

 creeping, underground stem (Fig. 67, C), which is covered 

 with brownish scales, and each leaf consists of a slender stalk, 

 reddish brown or nearly black in color, which divides into two 

 equal branches at the top. Each of these main branches bears 

 a row of smaller ones on the outside, and these have a row of 

 delicate leaflets on each side (Fig. 67, E). The stem of the 

 plant is fastened to the ground by means of numerous stout 

 roots. The youngest of these, near the growing point of the 

 stem, are unbranched, but the older ones branch extensively (0). 



On breaking the stem across, it is seen to be dark-colored, 



