6 MOSSES AND FERNS chap. 



function of the latter in most of them is to support the sporo- 

 phyte until it can take care of itself. 



When the lower Pteridophytes are compared with the more 

 specialised ones, a similar difference is found. In the lower 

 forms, like the Marattiacese and Equisetacese, the gametophyte 

 is relatively large and long-lived, and closely resembles certain 

 Liverworts. In these forms a considerable time elapses before 

 sexual organs are produced, and in artificial cultures of the 

 Marattiacese a year or more sometimes passes before archegonia 

 are formed. These prothallia, too, multiply by budding, much 

 as the Liverworts do. In case no archegonia are fecundated 

 the prothallium may grow until it reaches a length of three or 

 four centimetres, and resembles in a most striking manner a 

 thallose Liverwort. In such large prothallia it is not unusual 

 for more than one archegonium to be fecundated, although 

 usually only one of the embryos comes to maturity, and the 

 prothallium may continue to live for some time after the 

 sporophyte has become independent. Usually, however, as 

 soon as an archegonium is fertilised, the formation of new ones 

 ceases, and as soon as the sporophyte is fairly rooted in the 

 ground the prothallium dies. 



In most of the lower Pteridophytes the prothallia are 

 monoecious, but in the more specialised ones are markedly 

 dioecious. When this is least marked the males and females 

 differ mainly in size, the latter being decidedly larger; in the 

 more extreme cases the difference is much more pronounced 

 and is correlated with a great reduction in the vegetative part 

 of the gametophyte of both males and females. This reaches 

 its extreme phase in the so-called heterosporous forms. In 

 these the sex of the gametophyte is already indicated by the 

 character of the spore. Two sorts of spores are produced, large 

 and small, which produce respectively females and males. In 

 all of the heterosporic Pteridophytes the reduction of the vege- 

 tative part of the gametophyte is very great, especially in the 

 male plants. Here this may be reduced to a single quite 

 functionless cell, and all the rest of the plant is devoted to the 

 formation of the single antheridium. In the female plants the 

 reduction is not so great; and although sometimes but one 

 archegonium is formed, there may be in some cases a consider- 

 able number, and owing to the large amount of nutritive 

 material in the spore, in case an archegonium is not fertilised. 



