4 ON THE HAIRS OF PLANTS. 



vilves being double. I have also by observation noted tie 

 hairs always allotted for certarn purposes ; but there are 

 many uses it is not possible even to guess at. Innumerable 

 The various indeed are tlie offices these hairs perform. To shade from 

 uses of the jjght and heat, to convey moisture, to decompose water, to 

 catch and secure the drops of rain as they fall, and select the 

 deiy from the atmosphere, I have often seen them do; but 

 these, I conceive, are but a small part of the offices they daily 

 execute: when an instrument is wanted for the several pur- 

 poses of carrying moisture to the plants, catching the rain 

 drops on their points, and defending the back of the leaf 

 from the sun's rays, a simple kind of hair is generally used, 

 particularly found on the leaves of trees, as represented 

 PI. [, fig. 1. This is merely a managed vacuum, which 

 draws the water into the vessel, and thence lets it into the 

 pabulum of the leaf. It is well known, that the backs of 

 most leaves will not bear the scorching sun : and nature has 

 peculiarly formed and adapted the spiral wire, to turn the 

 The reason leaf if so directed. It is not from any great difference in 



leaves will not ^jji^g f^j. ^q^}^ cuticles are most frequently alike on each 

 bear the sun . . 



on their under side of the leaf; being both composed in part of this clear 

 tiirtaces. skin; but the one is pressed down on the pabulum, and is 

 always therefore moist : while the other stands much above 

 it, and, if heated, would soon dry up, peel off, and thus cause 

 the decay of the leaf. When leaves are to be defended from 

 heat alone, and no other purpose to be answered, then, (as in 

 coltsfoot and many other very wet plan ,,) the hairi are form- 

 ed like a ribbon with a quantity of threads woven round 

 them, and wholly without moisture. But in those which 

 contain moisture, all the different pipes have at the bottom 

 a contrivance for the entrance of the water into the pabu- 

 lum. This perfect mechanical process 1 have several time* 

 witnessed and described as the dew drops entering the cuti- 

 cle. See tig. 2, in which the thread a contracts or loosens 

 to admit or retain the water. When from a long continu- 

 ,.: ance of sunsliine and dry weather in February or March, 

 when the buds of trees are enlarging, and of course much 

 humidity is required for their preservation, a quantity of 

 Occasions! hairs will be suddenly feen covering all the buds in various 

 hairs bring directions, the sun'eracking the scales, and all the apertures 

 , / filled 



