182 ABSORPTION OF MOISTURE PROM THE ATMOSPHERE. 



to shine with great power; and as no rain fell, it was much 

 feared that the young plants, not having length enough of root 

 to penetrate deep into the soil, would be starved for want of 

 nourishment. This was supplied to them, however, in two ways 

 partly by the dews, which, in consequence of the clearness of 

 the nights, were heavy ; and partly by the action of the powerful 

 sun upon the deeper part of the soil, which had been drenched 

 (as it were) with moisture by the rain of the preceding autumn, 

 so that when it became heated, it sent up a large quantity of 

 vapour, which was probably absorbed both by roots and leaves. 



278. This absorption of fluid by the leaves, appears to take 

 place chiefly through the membrane of the cuticle ; but more 

 particularly by the downy hairs, which seem to act as so many 

 root-fibres. They are chiefly developed, as already stated 

 (, 96), upon plants which grow in situations in which they are 

 much exposed to light, and to a dry atmosphere ; whilst the 

 same species in damp shady situations will not be furnished with 

 any. It has been noticed that they lift up their points, and 

 separate from one another, at the approach of the evening dew, 

 which collects in minute drops around them ; and that they fall 

 down again, and form a layer of minute cavities on the cuticle, 

 as soon as the heat of the sun begins to be perceived. On com- 

 paring the increase in weight when exposed to dew, in plants 

 thickly furnished with hairs, and possessing few or no stomata, 

 with that manifested by plants having a smooth surface and 

 many stomata, it is seen that the former is much the greatest ; 

 and that it also surpasses, in about the same proportion, the 

 weight gained by immersing the footstalk in water. Thus two 

 heads of Marrubinm vulgare (Common Horehound), the original 

 weight of which was 15 grains each, were placed, one with its 

 stalk in the water, and the other in a place exposed to dew, for 

 a night; the first was found to have gained 2 grains, and the 

 second 5 grains. Both were exposed to dew during the next 

 night; and on the following morning they each weighed 23 

 grains, having both gained 8 grains, of which the first had 

 acquired 6 in that night. A withered stem of Cerastium 

 Alpinmn (Alpine Chickweed) weighing 5 grains, gained 6 grains 

 by exposure to dew for two nights. 



