6l2 



ECOLOGY 



the cushion mosses is the peat moss, Sphagnum, in which capillary 

 phenomena are facilitated further by drooping lateral branches (fig. 

 977), and especially by the presence of dead air-containing cells with 

 porous, fiber-thickened walls intercalated regularly among the syn- 

 thetic cells (fig. 899) . Not only do these dead cells fill with great rapidity, 

 but the presence of porous cell walls in the stem gives the plant an in- 

 ternal capillary system which sup- 

 plements the external capillary system 

 possessed in common with other 

 cushion mosses. 



Leucobryum much resembles Sphagnum, 

 except that porous dead cells form external 

 absorptive layers comparable to an absorp- 

 tive epidermis (as in orchid roots), the 

 chlorophyll-bearing cells occupying the 



Jl 



FIG. 899. A surface view of a part 

 of a leaf of Sphagnum, showing the nar- 

 row, elongated living cells (c) containing 

 chloroplasts, and the larger, colorless 

 dead cells (h) with their spiral thicken- 

 ings and pores (p~) ; highly magnified. 

 From COULTER (Part I). 



FIG. 900. A cross section of a leaf of 

 Leucobryum, showing the central chlorophyll 

 layer (c) with its chloroplasts, and the peripheral 

 layers (e) whose cells communicate with one 

 another by means of the pores (h) that per- 

 forate the walls ; highly magnified. 



median region of the leaf (fig. 900). The dry dead cells cause the characteristic 

 white aspect of the moss (whence the name, Leucobryum, i.e. white moss). Inter- 

 esting capillary phenomena of quite another sort are exhibited by Polytrichum, 

 a moss whose leaves become closely appressed to the stem during dry weather; 

 if the base of such a stem is placed in water, the leaves open out almost instantly by 

 reason of the capillary ascent of water along the dry sheathing leaf bases and between 

 the chlorenchyma plates of the leaves (figs. 901, 902). Doubtless much water enters 

 in this manner, thus supplementing ordinary leaf and rhizoid absorption. 



As in the air roots of orchids, the absorptive process in lichens and in 

 cushion mosses has two phases, the first being a capillary phenomenon, 





