﻿272 Evans: Thegexus Plagiochasma 



involve certain difficulties. He emphasizes the improbability of 

 having the male and female inflorescences morphologically dif- 

 ferent in a single genus, such as Reboulia, or in a group of related 

 genera, and he brings out the fact that if the carpocephalum of 

 Plagiochasma is really a dorsal outgrowth, as Leitgeb claimed, then 

 its scales would have to have a different significance from the 

 very similar scales found on a carpocephalum which represents a 

 branch system. In the latter case the scales are modified ventral 

 scales, homologous with those found on an ordinary thallus. In 

 Plagiochasma, however, the scales if considered homologous with 

 the ventral scales would imply an inversion of the dorsi-ventrality 

 of the thallus to account for their dorsal position; if considered 

 morphologically different from the ventral scales, they would have 

 to be regarded as developed from structures normally present on 

 the dorsal surface, such as the hairs found in certain species of 

 Riccia, or the slime-papillae which occur in the cupules of Mar- 

 chantia. Neither of these interpretations seems plausible. 



Goebel shows further (8, pp. 85-88) that these difficulties of 

 interpretation are avoided if such genera as Reboulia and Plagio- 

 chasma are considered as reduced from the more complex Mar- 

 chantiaceae rather than as primitive forms from which the more 

 complex genera have been evolved. In Marchantia, for example, 

 where the complexity perhaps reaches its highest expression, both 

 the androecium and the carpocephalum represent stalked branch- 

 systems, the sexual organs arising in acropetal succession on the 

 evident branches. A first indication of reduction appears when 

 the branches lose their distinctness although the acropetal succes- 

 sion of the sexual organs remains apparent. This is seen in the 

 androecium of Preissia. A second indication is a reduced develop- 

 ment of the stalk, leading to its final disappearance, as seen in the 

 androecia of Conocephalum and its allies. A third step in the 

 reduction, according to Goebel's account, is a change in the posi- 

 tion of the inflorescence, which is perhaps caused by a very early 

 appearance of an apical innovation. This may occur even in 

 vhich retain their stalks. In many cases such an 

 lot started until the inflorescence is already estab- 

 lished, when it shows its character clearly as a new shoot broaden- 

 ing out from a narrow base. Such a condition is evident in the 



