American Species of Marchantia. 209 



records of morphological importance are mostly in the form of 

 scattered notes, and these are often to be found in taxonomic 

 treatises. 



In the present paper the morpholog}- of Marchantia will be 

 treated largely from the standpoint of the taxonomist. In other 

 words the parts of the plant which yield the most distinct 

 and constant specific characters will be primarily considered. 

 These parts include the epidermis and the epidermal pores, the 

 compact ventral tissue, the ventral scales, the rhizoids, the 

 receptacles, and the cupules. The photosynthetic layer, the sexual 

 organs, and the sporophyte, although yielding important generic 

 characters, are less helpful when the individual species are con- 

 sidered. For the sake of completeness, however, a brief account 

 of the sporophyte will be included. 



The flat thallus of Marchantia is of the usual prostrate dorsi- 

 ventral type and branches repeatedly by forking. It varies con- 

 siderably in size and in thickness in certain species, so that 

 measurements of its various dimensions have to be employed with 

 caution. At the same time some of the species are distinctly 

 larger than others. The growth of the thallus is normally unlim- 

 ited until the sexual branches or receptacles (see Fig. 9, A, B) 

 are produced. These represent the erect prolongations of 

 prostrate branches and are limited in growth. The inflorescence 

 is dioicous throughout the genus. \"egetative reproduction is 

 carried on by means of discoid gemmae, which may be formed 

 on either male or female individuals and which apparently do not 

 interfere with the growth of the plant. 



The thallus shows clearly the usual dififerentiation into an 

 epidermis, a photosynthetic layer and a compact ventral tissue 

 bearing scales and rhizoids. The photosynthetic tissue consists 

 of a single layer of large air-chambers separated from one another 

 by continuous plates of cells. Each air-chamber is connected 

 with the outside by a single pore in the epidermal roof. From 

 the floor of the chamber arise numerous short rows of green 

 cells, subspherical in form and freely exposed to the air of the 

 chamber. The rows, which are simple or branched, are mostly 

 from two to five cells long and the uppermost cells, except in 

 the vicinity of the pore, are usually attached to the epidermis. 

 The air-chambers vary greatly in size, not only in different species 

 but often in different parts of an individual thallus. 



