Cavers : \ofes on Yorkshire Bryophytes. 



209 



(Plate Fig-, i, B. ). The formation of such branches is ahnost 



invariably seen after a female receptacle has been developed. 



The thallus varies considerably in external form and in 

 colour, this species being- the most polymorphic of the British 

 Marchantiacece. When g-rowing in moist and shaded situations, 

 the thallus branches freely at the apex, and its broad spreading 

 lobes are bright green in colour. Plants growing in drier places, 

 especially where exposed to full light, are deeply tinged with 

 purple, and the branches are shorter, broader, and more fleshy 

 than in the case of shaded plants. 



Fig. 2. — A. — Part of a transverse section of the thallus, showing- the dorsal zone of g-reen 

 air-chamber tissue and the ventral zone of compact colourless tissue. On the midrib 

 are shown the ventral scales in section, the small tuberculate rhizoids, and the large 

 smooth-walled rhizoids. X 35. B. — A ventral scale, showing- two long- narrow append- 

 ages. X 35. C. — Part of a scale, showing- one of the appendages, several marginal 

 mucilage-hairs, some of the oil-cells (shaded), and some tuberculate rhizoids springing- 

 from the base of the scale. X 150. D. — Part of the dorsal epidermis in surface view from 

 above, showing a pore surrounded by six concentric rings of cells, the walls of which, as 

 in the rest of the epidermis, show ang-ular thickenings. X 150. E. — Part of A, showing- 

 the structure of the lateral reg-ion (lamina) of the thallus ; the compact ventral tissue is 

 here reduced to two layers of cells. X 150. 



1904 July I. , o 



