198 Evans: Notes on genus Herberta 



old. According to Goebel* excessive thickening of the cell walls 

 in the bryophytes, which occurs especially in xerophilous species, 

 is significant on account of the great power of imbibition which 

 these walls possess. They readily absorb and retain water, and 

 the rapidity with which most species of Herberta regain their 

 normal appearance and consistency, when a dry tuft is deluged 

 with water, is a striking phenomenon. In the cauline portions of 

 the plant the cells are all elongated, although they never assume 

 a truly prosenchymatous character. The outer or cortical region 

 is distinguished from the inner or axial region by its deeper color 

 and more strongly thickened walls. In the secondary stems the 

 thickening at first takes place in the outermost layer of cells, the 

 process continuing until the cell cavities become reduced to narrow 

 canals (Text fig. 2). The thickening is largely restricted to the 

 outer or superficial walls and the layer presents the appearance 

 of an epidermis with well-developed cuticle. In some cases the 

 entire cortex consists of this outermost layer, the walls of the 

 remaining cells showing only a slight degree of thickening (Text 

 FIG. 26). This, however, is very unusual. In the majority of 

 cases two or more additional layers take part in the formation of 

 the cortical region; their walls become as markedly thickened as 

 those of the outermost layer, the only difTerence being that the 

 thickening is deposited uniformly on all the walls (Text fig. 3). 

 Even in the central region the thickening of the walls is sometimes 

 very pronounced, though never so much so as in the cortical region. 

 Since, moreover, the central cells are larger than the cortical cells, 

 their cavities remain distinctly wider. The cells of the stem are 

 everywhere connected by pits, which show clearly in both transverse 

 and longitudinal walls (Text figs. 3, 4). They are just as evident 

 in the superficial layer as elsewhere, the cells of this layer being 

 connected with one another both longitudinally and tangentially, 

 as well as with the cells of the layer next within. There is nothing, 

 in fact, to indicate any essential difference in function between 

 the superficial layer and the other cortical layers. In spite of the 

 fact that the outer walls simulate a cuticle so strongly, they have 

 the same powers of imbibition as the other walls and simply form 

 part of the imbibing system of the stem. In the rhizomes the 



* See Organographie der Pflanzen, 2d ed. 553. 1915. 



