﻿Evans: The genus Plagiochasma 273 



carpocephalum of Reboulia and also in the curious Plagiochasma 

 articulatum, according to Kashyap's account (15, p. 320). If, 

 however, the innovation starts much earher it might seem to 

 continue the growth of the thallus without an interruption and 

 thus to displace the inflorescence to an apparently dorsal position. 

 The latter would still represent a modified shoot, the scales would 

 still be homologous with ventral scales, and a thallus which showed 

 a median dorsal row of such inflorescences would really be a 

 sym podium. The change in position just described is clearly seen 

 in the androecium of Reboulia and in both the male and female 

 inflorescences of most species of Plagiochasma. In the case of the 

 androecium of Bucegia, recently investigated by SchiflFner (30), 

 all gradations in the process have been demonstrated. 



It is perhaps not necessary to assume that the carpocephalum 

 in Plagiochasma actually involves the growing point of the thallus 

 and that a new growing point must be differentiated before the 

 growth can be continued. If the important part played by 

 intercalary growth in the development of the carpocephalum is 

 taken into account such an assumption becomes superfluous. 

 Even in such complex carpocephala as those of Marchantia the 

 growth of the stalk and of the upper part leading to the displace- 

 ment of the archegonia to the lower surface is intercalary in 

 character. In Reboulia, after the earliest rudiment of the carpo- 

 cephalum is laid down in the young segments derived from the 

 growing point, the later development is entirely intercalary, as 

 Leitgeb himself admits (18, p. 31), and the growing point, carried 

 upward by the elongating stalk, plays no part whatever in the 

 process. In Plagiochasma the first beginning of the carpocephalum 

 is developed from young segments just as in Reboulia, and from 

 this rudiment the mature structure develops by intercalary growth 

 in the same way. The mere fact that the growing point is not 

 carried upward by the elongating stalk but continues directly 

 the growth of the thallus does not invalidate the homology of this 

 carpocephalum with that of Reboulia. In both cases the structure 

 owes its initial stages to the activity of the growing point of the 

 thallus and its later stages to intercalary growth, and in neither 

 case does the growing point play any active part in these later 

 stages. Of course what is here said about the carpocephalum 

 would apply in most respects to the androecium as well. 



