REGULATION OF TRANSPIRATION 267 



ditions in their successful efforts to regulate transpiration. 

 Early in the morning the leaves are spread out flat, at 

 hot noontide they are folded lengthwise, while in the 

 evening they unfold, and become either flat or fluted. 

 In the Marram Grass (Psamma arenaria), which grows 

 on sand-dunes and needs to conserve water, the folding 

 of the leaf in sunshine and in dry winds is particularly 

 marked. The transpiring surface is folded inwards, and 

 the living tissues are protected by layers of cells, with 

 lignified walls which contain only air, as well as by a 

 resistant epidermis on the exposed surface, all com- 

 bining to form an excellent screen which restrains 

 evaporation. If one picks a piece of the Common 

 Hair-Moss (Polytrichum commune), which abounds in 

 boggy places, and holds it for a short time in the hand, 

 the leaves will be observed to close round the stem and 

 clasp it. This is obviously a natural effort to conserve 

 water by restraint of transpiration; the plant re-enacts 

 speedily, under the abnormal circumstances, a move- 

 ment which is habitual in a state of Nature when drought 

 threatens. It gives us an exhibition, as it were, of " a 

 ruling passion strong in death." 



Very much more might be written in regard to a 

 multitude of devices for the regulation of transpiration ; 

 indeed, we have touched but the fringe of the subject, 

 and anything like an exhaustive treatment of it would 

 demand a volume to itself. What has been said will 

 suffice to draw attention to a most interesting study, 

 which the reader may be disposed to follow out in 

 greater detail. 



On several occasions we have referred to water as a 



