BIOLOGY OF HAIRINESS 183 



of vegetation. The difference in conditions is striking 

 in a small space in the case of the marsh society, but it 

 occurs equally in any close community ; the range of the 

 different factors of the environment being different, for 

 example, in a forest and a heather moor. 



Returning to the case of Spiraea, we see that the lower and 

 radical leaves occur in an atmosphere in which evaporation 

 is much reduced, while upwards, as hairiness increases, the 

 evaporation is successively greater. Even if we take a single 

 leaf we see that the most exposed apical leaflet is the one 

 which tends to be most hairy. Even in the individual leaflet 

 the margin first becomes hairy, and this is the region most 

 liable to drought — in many leaves it withers first in keen 

 winds. 



Yapp was not successful in experimentally modifying, 

 to any considerable extent, the regular succession of hairiness. 

 No degree of drought could make the spring radical leaves 

 hairy ; not even a combination of low light intensity and 

 high humidity suppressed the pubescence of leaves normally 

 hairy ; at the most the number and size of the hairs were 

 reduced. 



All this forms a very striking parallel to the case of the 

 sun and shade leaves already dealt with, all the more so 

 because the hairy and glabrous leaves are also anatomically 

 sun and shade leaves. We might say that hairiness is here 

 an additional sun leaf character. We have the same 

 general relation between the character and the environmental 

 conditions in which it normally occurs ; and we have the 

 same limited plasticity shown by a leaf developing under 

 the action of the set of conditions, opposite to that in which 

 it would normally grow. It is supposed by Yapp that 

 diminution of water supply stimulates certain cells to 

 elongated growth — the epidermal cells which produce hairs, 

 the root hairs, and the palisade. Some experimental 

 evidence can be adduced in favour of this view, though it 

 cannot be said to advance much our understanding of the 

 mechanism of hair production. The fact that large changes 

 in the external conditions have so little effect makes the 



