CAPILLARY CELLS OF MOSSES 



235 



velamen takes place, if a root which is at first aerial subsequently 

 enters the ground. 



At this stage reference may be made to certain anatomical specialisa- 

 tions among Mosses, 123 which may be regarded as the physiological 

 counterparts of the velamen of aerial roots, inasmuch as in both cases 

 water is absorbed through aerial organs by means of capillary action. 



In the genus Leucobryum, and in the Leucobryaceae in general, the 

 leaf consists of a single median layer of green cells enveloped on either 

 side by one or more layers of large colourless cells devoid of proto- 

 plasmic contents, which communicate with one another and with the 

 outer atmosphere by means of large circular holes in their walls. The 

 photosynthetic tissue is thus enveloped in a system of capillary tubes. 

 So long as these capillaries are full of air, the leaf has a whitish 

 appearance hence the name Leu- 

 cobryum ; when the leaf is wetted, 

 however, the capillaries instantly 

 become filled with water, and the 

 green colour of the photosynthetic 

 cells becomes apparent. 



In the genus Sphagnum the 

 tubular photosynthetic cells of the 

 leaf are arranged in the form 

 of a network, the meshes being 

 occupied by colourless " capillary 

 cells" (Fig. 91). The walls of the 

 latter are stiffened by means of 

 annular or spiral fibrous thickenings, and in this respect display 

 a higher stage of adaptation than the thin-walled capillary cells 

 of Leucobryum. In the distal portions of some of the leaves the 

 annular and spiral thickenings expand, according to Kussow, into 

 broad plates or ribbons, which constitute a particularly effective 

 stiffening apparatus. The unthickened tracts of the walls are 

 perforated by large openings, which are usually circular in out- 

 line ; very frequently the margin of each hole is surrounded by 

 an annular thickening fibre. The stem of Sphagnum is likewise 

 provided with a capillary apparatus comprising from two to four of the 

 outermost cortical layers. The capillary cells resemble the analogous 

 elements of the leaf; both their transverse and their longitudinal walls 

 are perforated, so that the whole tissue can become saturated with 

 water in a very short time. As the individual cells are of considerable 

 size, relatively large amounts of water can be stored in this cauline 

 capillary tissue, while a certain amount of upward movement of the 

 water may also be effected by capillary action. We are therefore, in 



Fig. 91. 



Part of a leaf (from a lateral branch) of Sphag- 

 num cymbijolium, in T.S. : r, annular thickening 

 fibre of a capillary cell ; I, perforations in the 

 outer walls of the capillary cells ; c, photosyn- 

 thetic cells. After Russow. 



