258 



MOSSES AND FERNS 



CHAP. 



of the thallus and merges gradually into the wings, which are 

 also several-layered, nearly or quite to the margin. 



The very old prothallia sometimes branch dichotomously 

 (Fig. 132, B, C), and the process is identical with that in the 

 thallose Hepaticae. The two growing points are separated by 

 a median lobe in the same way, and the midrib with the sexual 



Fig. 132. — Marattia Z>oug-/asn (Baker). A, Prothallium about one year old, X2 ; B, the same pro- 

 thallium about a year later, showing a dichotomy of the growing point ; C, the same seen from 

 below, showing two archegonial cushions ( ? ) ; D, prothallium with young sporophyte, X 4 ; E, a 

 somewhat older one, seen from the side, X 4 ; r, the primary root. 



organs upon it forks with it, exactly as we find, for example, 

 the antheridial receptacle forking in Fivib7'iaria Californica (Fig. 

 I, A). Besides this form of branching, which is not common, 

 adventitious buds are produced upon the margin of the thallus 

 very frequently. These grow in precisely the same way as the 

 main prothallium, and after a time may become detached and 

 form independent plants ; or they may develop sexual organs 

 (mainly antheridia) while still connected with the mother 



