﻿282 Evans: The genus Plagiochasma 



external conditions. The variations in size and in inflorescence 

 have already been considered. With regard to color the thallus 

 is sometimes green throughout but is usually more or less pig- 

 mented with purple. The pigmentation may be confined to the 

 ventral scales, but it usually affects also the ventral surface from 

 the scales to the edge and a narrow border on the dorsal surface; 

 sometimes, in fact, the entire dorsal surface may be more or less 

 colored, although this condition is rare. 



The structure of the epidermis and its pores is also worthy of 

 comment (Fig. i). Not only do the epidermal cells vary 

 markedly in size, but their walls differ in thickness and the trigones 

 are sometimes conspicuous, with straight or convex sides, and 

 sometimes hardly to be made out at all. The waxy layer on the 

 cuticle, moreover, is sometimes so thick and opaque that it is 

 difficult to see the epidermal cells beneath. The epidermal pores 

 escaped the observation of the early authors, and even Nees 

 von Esenbeck (26, p. 45) and DeNotaris (5, p. 477) said that . 

 no pores were present. Apparently Voigt, in 1879, was the 

 first to detect them (40, p. 747, pi. g, /. 24, 25), In JP. Aitonia 

 he says that the pores are the smallest known to him, that 

 the cells bounding the opening differ from the other epidermal 

 cells only in their thickened radial walls, that the inner angles 

 of these cells are often cut off by walls thus forming two circles 

 (of four cells each), and that the inner cells of these circles are 

 often so small that they can easily be overlooked. In P. italicum 

 he says that the pores are larger, that the radial walls are scarcely 

 thickened, and that the inner cells of the two concentric circles 

 are almost always distinct. His figure of the surface view, which 

 is said to represent P. Aitonia, does not show the radial thickenings 

 very clearly, but his account of the two species which he dis- 

 tinguishes brings out the amount of variation to be expected in 

 P. rupestre. In the vast majority of cases the number of cells 

 around the openings varies from four to six. In very rare cases 

 three cells or more than six may be demonstrated. In the accom- 

 panying figures the great variations in the size of the opening, in 

 the thickness of the radial walls, etc., are shown. Apparently the 

 degree of thickness which the radial walls show varies with the 

 size of the trigones in the neighboring epidermal cells. In culti- 



