64 



STRUCTURAL BOTANY 



the growing-point, forms a pad or cushion several cells 

 in thickness, while the lateral portions or wings remain 

 one cell thick. 



The antheridia or male organs arise chiefly on the 

 older basal part of the prothallus and sometimes also on 



the wings. The female 

 organs or archegonia are 

 limited to the cushion. 

 In ordinary cases the 

 prothallus is monoe- 

 cious, bearing ' both 

 kinds of sexual organs ; 

 but this is not always 

 so. Male prothalli are 

 not uncommon, and are 

 generally of small size. 

 Sometimes a prothallus 

 at the earliest stage of 

 its development, while 

 still in the form of a 

 short filament, begins to 

 form antheridia. A 

 filamentous prothallus, 

 bearing male organs 

 only, is shown in Fig. 

 32, but still smaller 

 ones occur. 



Specimens with arche- 

 gonia only are rarer, and are of the ordinary form. 

 These variations are interesting, because they show how 

 the distinction of sex among the individual prothalli, 

 which has become fixed in Selaginella and other hetero- 

 sporous forms, appears occasionally as a more or less 



Fio. 32. Young male prothallus of 

 Male Fern, an, antheridia ; sp, 

 spermatozoids escaping ; r.h, root- 

 hairs. Magnified about 70 diameters. 

 ( After Kny.) 



