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THE BOOK OF NATURE STUDY 



To LESSEN TRANSPIRATION. As a general rule plants that 

 grow in dry situations are much more hairy than those belonging 

 to damp places. The different species of a genus illustrate this. 

 Crepis paludosa grows in damp places and is smooth, whilst 

 Crepis biennis and Crepis fcetida are hairy. Professor Henslow, 

 describing the plants of the desert near Cairo, notes the grey colour 

 of the leaves, and attributes it to the dense coating of hairs, which 

 conceal the green colouring matter. The effect of hairs is to lessen 



the amount of transpiration. This is 

 a most important function, for plants 

 that live in dry situations need to 

 retain as much water as possible, the 

 supply being very irregular. A cover- 

 ing of hairs prevents evaporation 

 from the surface of the leaf into the 

 air. This may be observed on hills, 

 which are always drier than the 

 adjoining valleys and on the seashore. 

 Many of the Hawkweeds and Hawk- 

 bits and Crepis have leaves densely 

 covered with hairs, to protect from 

 too rapid transpiration. In the 

 Mouse-ear Hawkweed the under sur- 

 face of the leaf is covered with 

 stellate hairs ; in dry weather the 

 leaf rolls up so that the under surface 

 is uppermost, and evaporation con- 

 siderably lessened. This decrease of 

 transpiration owing to the protection 

 of hairs is one of the best marked characteristics of alpine 

 plants. The Edelweiss and Cudweeds are instances that will 

 occur to every one. 



In this connection it is interesting to note the variation in 

 the degree of hairiness in the same species owing to a difference 

 in situation. The Restharrow (Ononis arvensis) is very variable. 

 When growing by the sea it is more hairy than when growing 

 inland. Similarly, it has been stated that Meadow-sweet, which 

 is usually smooth, develops hairs when growing in a dry situation. 



FlG. 43. A species of Crepis show- 

 ing hairiness of leaves. 



