CHAPTER XIII. THE PTERIDOPHVTA. 353 



like a cork-screw, and provided at its thinner end with numerous 

 cilia. 



The archegonia are usually borne near the growing or heart- 

 shaped end of the prothallium. They are flask-shaped bodies 

 with necks rather short, and curved backward or toward the base 

 of the prothallium. See Fig. 560. The mode of fertilization 

 and the development of the asexual plant from the fertilized 

 germ-cell, are essentially the same in the entire class. It has 

 already been described at the beginning of this chapter. 



The stems in a few instances, as in Pteris aquilina, have the 

 internodes rather long and not covered by the persistent leaf- 

 bases, but, in most instances, the nodes are crowded together, 

 and the leaf-bases completely obscure the stem, giving it a scaly 

 appearance. 



The leaves often bear hairs which are usually flattened and 

 sometimes conspicuous for their size, forming a brownish chaff 

 which, not infrequently, as in the Male Fern, completely invests 

 the young leaves. They are technically called palece. 



The sori, in their arrangement, usually bear a definite relation 

 to the venation of the leaves, and as this is different in different 

 groups, it affords an important means of distinguishing genera 

 and species. For example, in Pteris they are borne on the lower 

 margin along the terminations of the veins, and are protected by 

 the revolute margins of the leaf; in Hymenophyllum, they 

 occur on prolongations of the veins ; in Polypodium, they are 

 on the under surface at the extremities of short veins ; in the 

 Marattias, they are borne on a kind of cushion which is an out- 

 growth from the surface of a vein ; and in Acrostichum, they 

 occur anywhere on the lower surface, sometimes covering the 

 greater portion of it. 



In some species the sori are naked, that is, not enclosed by a 

 protecting membrane, but in many such a membrane is present, 

 and its form and structure often affords characters by which 

 groups are distinguished. In the Shield-ferns, for example, it is 

 a shield or kidney-shaped membrane ; in the Aspleniums it has 

 one edge attached to a vein while the other is free ; and in some 

 of the Cyathaceae it is capsular and completely encloses the 

 sporangia. 



The sporangia themselves differ considerably in different 



