American Species of Marchantia. 225 



initials are present among the cells of the scales, the appendages 

 alone being free from them. 



The antheridia arise in acropetal succession, the oldest being 

 formed near the center of the disc. In many species each ray 

 develops two distinct rows of antheridia, but in certain species, 

 such as M. polymorpha, the antheridia are more irregular in their 

 arrangement and each ray shows more than two indistinct rows. 

 The antheridia are borne singly in deep depressions with small 

 circular openings. The depressions extend down into the com- 

 pact ventral tissue, and are surrounded by the characteristic 

 air-spaces with their branched rows of photosynthetic cells and 

 dolioform epidermal pores. 



The stalk of the female receptacle develops more slowly than 

 that of the male receptacle and persists in an active condi- 

 tion until the sporophytes are mature. In the disc the division 

 into rays takes place just as in the male receptacle and the num- 

 ber of rays present is subject to similar variations. The arche- 

 gonia form groups and arise in acropetal succession, beginning 

 when the disc is very young; but, on account of the strong inter- 

 calary growth in the median region of the dorsal portion, the 

 archegonia are arched over and displaced until they seem to be 

 situated on the ventral surface of the disc. In this way the oldest 

 archegonia come to lie nearest the periphery of the disc and the 

 youngest nearest the stalk. Each group of archegonia contains 

 a variable number, arranged in two or three more or less definite 

 radial rows, and is derived from one of the growing regions of 

 the disc. 



In the East Indian M. geminata and its allies the groups of 

 archegonia are clearly situated underneath the rays of the 

 receptacle. These rays, therefore, are obviously homologous 

 with the rays of the male receptacles throughout the genus. 

 This condition, however, is very exceptional. In most species 

 of the genus, including all the American representatives, the 

 groups of archegonia alternate with the rays of the recep- 

 tacle and are situated in the sinuses between them. This 

 is caused by the rapid intercalary growth of the regions 

 between the growing points, the so-called "middle lobes" ; the 

 rays, accordingly, are formed by the middle lobes and are not 

 homologous with the rays of the male receptacle but rather with 

 the sinuses. These relationships are discussed at length by Leit- 



