Xl] EPIDERMAL HAIRS 85 



loving plants like Ferns, Begonias, and some others. It 

 must not be concluded that this is due to direct light- 

 reactions, however, for the chloroplasts are present as 

 colourless bodies around the nuclei, and in the guard- 

 cells of the stomata they attain their normal size and 

 green colour, even in the cases where they are lacking 

 in the ordinary epidermal cells. 



But although the cells rarely contain chlorophyll, and 

 are often quite colourless, it commonly happens that their 

 cell-sap is coloured red, more rarely blue or other colours, 

 and in many cases at least there can be no doubt that the 

 red sap serves as a screen against too intense illumina- 

 tion, or as an absorbent of certain rays of light used for 

 special purposes e.g. the frequent red colouring of many 

 young foliage leaves and shoots while the blues and 

 other colours in the epidermis of petals are of use to 

 attract insects. 



Various other substances, such as tannins, soluble 

 starch, &c., may also occur in the epidermis. 



Hairs of various kinds play such important parts 

 in the biology of plants that we may well regard them 

 as a distinct category of organs. 



A typical hair is an outgrowth of a single superficial 

 cell, and, in aerial organs of the higher plants, of a cell 

 of the epidermis. This definition, though it covers the 

 morphological idea of what a typical hair really is, must 

 not be pushed too far however, for hairs may arise from 

 other than epidermal cells (e.g. Root-hairs, the hairs on 

 Fern-prothallia), and more than one cell may be concerned 

 in their origin : moreover, it is impossible to distinguish 

 sharply between hairs and exactly similar organs in certain 

 lower plants which receive different names. Finally, hairs 

 may be formed internally in the body of a plant e.g. in 

 the lacunae of some Water-Lilies and Aroids. 



