THE COLTSFOOT 265 



{Petastites, Plate XLV.) produce very large leaves wlien 

 they grow in damp, sliady sites, but not nearly so large 

 when they are exposed to dry air and sunlight. Mention 

 of Coltsfoot reminds us of the clothing of hairs on both 

 surfaces of its young leaves, and on the under surface 

 of those which are full-grown. In Britain, the young 

 leaves appear early in the season, when they are subject 

 to considerable variations in temperature and heavy 

 rains; doubtless their hairy clothing serves not only to 

 retain heat, but also to prevent the stomata from being 

 choked by moisture, which would hinder the escape of 

 water vapour. These young leaves are also inclined at 

 an angle at which they can benefit most completely from 

 the incident rays of sunlight, which promote transpira- 

 tion and also encourage carbon-assimilation. The angle 

 of inclination of the full-grown leaves is not so high, and 

 as they attain their maximum growth in the height of 

 summer, they do not then need to retain heat by a thick 

 clothing of hairs on their upper surface, nor are the hairs 

 needed to ward ofi moisture on that surface because 

 there the stomata are few in number; but the under- 

 surface is well furnished with stomata and thickly clad 

 with hairs, the chief function of the latter evidently 

 being to ward ofE dew, which we must remember really 

 rises from the ground, and can as easily condense on the 

 under as the upper surface of a leaf. The necessity for 

 the hairy garb on the under surface of the Coltsfoot leaf 

 becomes the more evident when we appreciate the fact 

 that this plant grows in colonies whose leaves cover and 

 shade considerable areas of ground, and the air beneath 

 this leafy canopy is always humid, even on days of 

 bright sunshine. 



34 



