ANTHOCYANIN IN EPIDERMAL CELLS 117 



in the case of Corylus Avellana the corresponding period would be 

 barely three-quarters of an hour, and in that of Pyrus communis only 

 forty minutes. It follows from these measurements that the ordinary 

 epidermis would be hopelessly ineffective as a water-reservoir, if 

 stomatic transpiration were always going on. Under normal con- 

 ditions, however, the epidermis is never called upon, in the case of 

 leaves that can close their stomata, to make good the large loss of 

 water which the photosynthetic tissues suffer on account of stomatic 

 transpiration ; for Von Mohl, Leitgeb and others have shown that, as 

 soon as a leaf begins to wilt, the guard-cells at once begin to close 

 the stomatic orifices and thus to put a stop to stomatic transpiration. 

 Cuticular transpiration on the other hand is usually so small in 

 amount, even in bright sunshine, that the resulting loss of water can 

 be borne by the epidermis alone for a very considerable length of time. 

 Slightly withered leaves of Horse-Chestnut, Hazel and Pear lose so 

 little water, at any rate during the first twenty-four hours, that the 

 half-collapsed epidermal cells can make good the loss for a period 

 amounting in the three instances to 16i, 6 (nearly) and 7 hours 

 respectively. During these periods, therefore, the photosynthetic 

 tissue need not suffer any actual loss of water, always provided that 

 the cell-sap does not become so concentrated in the transpiring epidermal 

 cells as to abstract water from the adjacent elements of the mesophyll. 

 There are, therefore, good reasons for regarding even the ordinary flat 

 epidermal cells as reservoirs of water which are able to cope effectively 

 with purely cuticular transpiration. 



The epidermal cells of vegetative organs occasionally contain pig- 

 ments dissolved in the cell-sap ; anthocyanin (erythrophyll), which 

 varies in hue from bright red to bluish-violet, is perhaps the most 

 widely distributed of these colouring matters. Where this coloration 

 has any physiological significance at all, it is probably always connected, 

 in some way, with the relations of the plant to light. 64 In many cases 

 the coloured epidermis appears to act as a screen which protects the 

 underlying tissues against intense illumination and its secondary con- 

 sequences, such as rapid destruction of chlorophyll (according to 

 Wiesner) or excessive respiration (according to Pringsheim). This 

 view is supported by a variety of circumstances. Thus Von Mohl long 

 ago drew attention to the frequent red coloration of young shoots and 

 seedlings, the nascent chlorophyll of which is particularly liable to 

 destruction by light. A great many evergreen leaves acquire a reddish 

 colour in winter time owing to the formation of anthocyanin ; in this 

 case the chlorophasts require special protection during winter against 

 the injurious action of light, because no appreciable regeneration of 

 chlorophyll takes place at the low temperatures which prevail at that 



