346 FORM AND POSITION OF THE TRANSPIRING LEAVES AND BRANCHES. 



A process, very similar to the opening and closing of grass-leaves, is also to be 

 observed in the true mosses, in all species of the genus Polytriehum, and in some of 

 the Barbulas. The peculiar structure of the leaves of these mosses has been already- 

 treated of. In addition to the description there given, it may be mentioned that the 

 ridges of thin-walled green cells, which are present on the upper surface of such a 

 leaf (see fig. 89), only remain exposed to currents of air as long as this air possesses 

 the requisite degree of humidity; that is to say, the blade of the leaf from whose 

 upper surface the bands project only remains expanded while that is the case 

 (fig. 89 2 ). 



As soon as the air becomes dry, the lateral portions of the leaf-blade bend 

 upwards, and envelop the green ridges like a mantle (fig. 89 1 ). These are then 



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ssbe 



Fig. 89. — Folding of Moss-leaves. 



Transverse sections through the leaf of a Polytriehum (Polytriehum commune), i The leaf dry and folded. 

 2 The leaf damp and open; x85. 



inclosed in a hollow chamber, and only retain communication with the surrounding 

 air by a narrow slit above, which is left open between the inflected leaf -margins. 

 But here again it should be noticed that the highest cells in each ridge are strongly 

 thickened on the part turned towards the opening, which doubtless helps to lessen 

 transpiration. The opening and closing of the Polytriehum takes place very rapidly. 

 By repeated hygrometric changes in the air, the process may be performed naturally 

 several times in a single day. In Polytrichacese, which have been plucked while their 

 leaves were open, the closure is seen to be completed, in dry air, in a few minutes. 

 Dead and withered leaves are always closed, and never reopen, even when kept 

 damp for a long time — from which it may be concluded that the mechanism of the 

 opening and closing cannot be due to a simple hygroscopic phenomenon. Probably, 

 the same mechanical forces come into action as produce the folding of leaves of 

 grasses; but the process in moss-leaves is much more complicated, since it consists, 

 not merely in the upward inclination of the leaf-edges, but also in an upward curva- 

 ture and spiral twisting of the whole leaf. 



