2i8 Alexander W. Evans, 



cells will occur end to end and sometimes even longer groups 

 or strands are formed, perhaps corresponding with Stephani's 

 "strands of sclerenchyma." The walls of the cells are thick 

 and pigmented, showing that their functions are primarily 

 mechanical, but Goebel finds that the cavities sometimes contain 

 starch-grains. Sclerotic cells occur in several American species, 

 such as M. paleacea, M. chenopoda (Fig. 20, A, B) and M. dotnin- 

 gensis, and agree closely with Goebel's description. Whether 

 they are always produced by the species where they have been 

 detected is perhaps a question. Cavers-^ calls attention to the fact 

 that Preissia quadrata, when grown indoors in a moist atmos- 

 phere, fails to develop thick-walled cells, and in all probability 

 the formation of the similar cells in Marchantia is influenced by 

 environmental conditions. In any case, however, the presence 

 of sclerotic cells is associated with certain definite species. 



3. Ventral Scales 



The ventral scales in Marchantia exhibit considerable diversity, 

 not only when different species are compared but also when an 

 individual species is considered. This is due to the fact that 

 each species produces at least two distinct kinds of scales, only 

 one of which bears appendages. In the other -genera of the 

 Marchantiaceae the scales with appendages are the only kind 

 produced. 



Taylor^® was apparently the first to observe that the scales in 

 M. polymorpha were not all alike. He distinguished three dif- 

 ferent kinds, and these are described at length by Leitgeb,^" who 

 designates them as median, laminar and marginal scales, respec- 

 tively. The median scales are attached by a long line, which 

 begins near the axis of the thallus, then extends almost longi- 

 tudinally and finally curves gently outward, reaching perhaps 

 half way to the margin. The scales are at first very narrow 

 but become abruptly dilated in the outer part ; here on each scale 

 the characteristic appendage is attached, strongly contracted at 

 its junction with the scale and then abruptly dilated into an 

 orbicular expansion, rounded to apiculate at the apex (Fig. i). 



^' Contrib. to the Biol, of the Hepat. 28. Leeds and London, 1904. 

 ^ Trans. Linn. Soc. 17 -.377. 1835. 

 '"Unters. iiber Leberm. 6: 114. 1881. 



