210 



BOTANY 



from the ventral tissues. The air-chambers may be irregular, or they may be 

 clearly circumscribed, and visible to the naked eye as polygonal areolations 

 upon the dorsal surface of the thallus. The pores in the epidermis may be 

 openings surrounded by radiating epidermal cells, or there may be a definite 



stoma, composed 

 A of a chimney-like 



series of super- 

 imposed tiers of 

 cells, the lower 

 ones much en- 

 larged and capable 

 of closing the open- 

 ing. This form of 

 stoma is especially 

 well developed 

 upon the arche- 

 gonial receptacles 

 (Fig. 168). 



The ventral 

 scales in -the Mar- 

 chantiaceae arise 

 singly on either 

 side of the midrib, 

 and are not formed 

 by the splitting of 

 a continuous la- 

 mella, as in Riccia. 

 Oil-bodies, borne 

 in special cells, and mucilage-ducts, occur in some species. 



Gemmae. Gemmae of a peculiar type (Fig. 169) occur in Marchantia and 

 Lunularia. They are flattened buds which are borne in large numbers in special 

 receptacles, cup-shaped in Marchantia, semicircular in Lunularia. They arise 

 from single epidermal cells, which divide transversely into a short basal cell 

 which remains undivided, and a terminal cell which by repeated divisions gives 

 rise to the bud. This becomes lenticular in form, with an indentation on each 

 edge, so that it is fiddle-shaped. These indentations mark two growing-points, 

 and when the bud falls upon the earth they grow out in opposite directions into 

 two shoots. The buds are detached by the swelling of a mass of mucilage 

 secreted by small glandular hairs growing with them. Both surfaces of the bud 

 are alike, and over these are scattered colorless cells which grow into rhizoids 

 on coming into contact with the ground. Whichever surface of the bud falls 

 downward becomes the ventral surface of the young plant, and the upper sur- 

 face develops into the characteristic dorsal tissues. 



Sex-organs. The sexual organs, which closely resemble those of Riccia, are 

 borne in groups upon more or less modified parts of the thallus. The plants are 

 either monoecious or dioecious. The antheridial receptacle may form a cushion 

 upon the dorsal surface of an ordinary shoot, as in Fimbriaria (Hypenantron), 

 or there may be special antheridial branches, as in Targionia and Marchantia 

 (Fig. 166). 



The archegonia are always borne upon more or less modified shoots, which 

 more commonly develop into the mushroom-shaped receptacles (Fig. 164) so 

 familiar in Marchantia and other genera. These receptacles may represent a 

 single branch, or they may be composed of a system of short branches. 



FIG. 168. Fimbriaria Californica. A-C, development of the 

 pores upon the receptacle, longitudinal sections. D, surface- 

 view of a pore. 



