NOTES ON LEAP-SECTIONS OF POLYTRICHUM. 187 



The position of the lamellae, when the leaves are expanded in 

 the moist condition, is evidently an adaptation to their function 

 as the assimilatory tissue-system of the plant. They then stand 

 vertically upon the expanded leaf, with their surfaces in line 

 with the incident light-rays. The arrangement, in fact, strongly 

 recalls the " palisade cells " in the ordinary bifacial leaf of 

 dicotyledons. 



Like all mosses, the species of Poly trichum are liable to loss of 

 water during dry weather, and are well adapted to endure 

 desiccation. Their elaborate leaf-structure presents some inter- 

 esting points of adaptation to such conditions. In all the species 

 there is a broad sheathing leaf-base, from which the lamellae are 

 for the greater part absent, the lamina being here expanded, 

 and consisting of thin-walled hyaline cells. In some species — as, 

 for example, in P. juniperinum — this sheathing base is not very 

 extensive ; and, in that case, the marginal ring is broader, and folds 

 over the lamellae in the dry condition. In other cases, the leaves 

 become closely adpressed to the stem, thus affording a similar 

 protection to the lamellae. This movement of the leaves is due in 

 large measure to contraction of the sheathing base, owing to the 

 withdrawal of water from the thin-walled tissue of which it is 

 composed, but also in part to contraction of the lamellae them- 

 selves from a similar cause. The cells of the lamellae are mostlv 

 thin-walled, and readily absorb moisture, both in the liquid form 

 and as atmospheric vapour ; but the external walls of the cell-row 

 which form the upper free edge of the lamellae are strongly and 

 characteristically thickened. When the leaves dry up, the 

 lamellae became closely packed together, the only part exposed to 

 the air being thus this thickened upper edge. 



The thickening of the outer row of cells in the lamellae differs 

 in form in the different species. Thus in P. urnigerum the wall 

 is covered with wart-like papillae. This is also the case in P. 

 alpinum. In P. juniperinum there is a strong ridge, of cellular 

 thickening on the upper edge of the cell-row, and two lateral 

 ridges less strongly marked, giving the appearance of a roughly 

 cruciform cell, as seen in section. P. piliferum agrees in form 

 with P. juniperinum, but the lateral ridges are almost obsolete. 

 In P. commune the edge of the lamellae is grooved longitudinally, 

 giving the appearance of a bicuspidate cell in section. 



