2IO Alexander W. Evans, 



I. Epidermis and Epidermal Pores 



The ordinary epidermal cells are fairly uniform throughout 

 the genus and it is doubtful if they offer any very trustworthy 

 differential characters. Their size often varies markedly on an 

 individual thallus and may be directly affected by differences in 

 external conditions. Although the cells are usually colorless or 

 pale they sometimes produce chloroplasts in abundance. In the 

 majority of cases they are arranged in a single layer, but in 

 certain species at least, such as 71/. chenopoda (Fig. 19, E) and 

 M. paleacea (Fig. 8, D), the epidermis may be two cells thick in 

 parts of its extent. The walls may vary considerably in thick- 

 ness, but they are rarely very firm and are destitute of distinct 

 trigones. 



Cells containing oil-bodies, cells containing slime, and minute 

 surface papillae are sometimes found in the epidermis. The cells 

 containing the oil-bodies are usually distinctly smaller than the 

 neighboring cells and are easily distinguished by their granular 

 contents, which nearly or quite fill the cell cavities. In M. cheno- 

 poda these cells are not infrequent and do not seem to be 

 restricted to any definite part of the thallus; in M. polymorpha 

 they occur near the margin and seem to be absent elsewhere ; 

 while in certain other species there are apparently no cells of 

 this character in the epidermis. 



Epidermal cells containing slime are, according to our present 

 knowledge, restricted to M. chenopoda. The slime-cells are 

 scattered about in the epidermis and always occur in regions 

 where the epidermis is two cells thick, being situated in the inner 

 layer (Fig. 19, L). They are much larger than the surrounding 

 epidermal cells and strongly compress those of the outer layer. 

 When a piece of the epidermis is examined from above the slime 

 cells are seen to be covered over by these compressed cells. 

 Apparently Voigt* was the first to observe the slime-cells, 

 although he failed to recognize their true character. The much 

 larger slime-canals in Conocephalnm conicum (L.) Dumort. 

 were soon afterwards described by Goebel," and Leitgeb^" 

 pointed out that the slime-cells of M. chenopoda were of the same 



' Bot. Zeit. 37 : 733- 1879. 



" Arb. Bot. Inst. Wiirzburg 2 : 531. 1880. 



^"Unters. iiber Leberm. 6:16. 1881. 



