CONDITIONS OF SIMPLE EVAPORATION. 169 



of the surrounding air. Every one knows that warmth is of 

 great assistance, in drying moistened substances of any kind ; 

 and this results, from its promoting the conversion of the water 

 into vapour. It may easily be observed, too, that a damp atmo- 

 sphere retards the process ; and the air sometimes has so large a 

 quantity of vapour suspended in it, that it deposits it as a dew 

 upon dry substances, instead of raising fresh moisture from damp 

 ones. 



255. Now the living fabrics of Plants are subject, like all 

 other moist substances, to the loss of fluid by evaporation ; and 

 this would take place under the conditions just mentioned, from 

 all the parts which have this character, were it not for the pro- 

 tection afforded by the cuticle. This membrane, as formerly 

 stated, covers the whole surface of every plant, which is exposed 

 to the air ; and, from its peculiar nature, it is not liable to be 

 thus influenced by heat or dryness of the atmosphere ; so that it 

 effectually protects, from the undue influence of these agents, the 

 soft tissues beneath. The difference which results from the pre- 

 sence or absence of this cuticle, may be well seen, by comparing 

 the long-continued freshness of the leaf of any Flowering-plant 

 which is kept in the dark (so that its exhalation, or transpiration 

 of fluid through the stomata, as presently to be explained, is 

 prevented,) with the rapid shrivelling of the frond of a Sea- Weed, 

 or of any Flowering-plant that naturally grows beneath the 

 water, when equally exposed to the influence of a warm and dry 

 atmosphere. And, as already noticed, the cuticle is almost in- 

 variably found to be the thickest and firmest, in plants which 

 frequent very hot and dry situations. 



256. Nevertheless the cuticle does not entirely check eva- 

 poration ; but this takes place from the surface of a dead plant, 

 or of any portion of one, as well as from one in the most active 

 vegetation. The shrivelling of Apples long kept, and the loss 

 of weight of Potatoes, are examples of this slow and gradual 

 change. It may be stated, then, that Plants, like other moist 

 soft substances, are liable to part with some portion of their fluid 

 by evaporation, especially when exposed to a warm and dry 

 atmosphere ; but that the amount of this loss is far too small, to 



