ACTION OF LEAVES. 49 



71. Although perspiration thus appears to be prin- 

 cipally excited by the solar rays, and to be in a given 

 plant in proportion to their intensity, yet we are not 

 authorised in concluding that perspiration is not 

 increased or diminished by the medium in which a 

 plant grows. Immersed in water, perspiration is 

 necessarily arrested ; in an ordinary atmosphere, 

 it will be in proportion to the quantity of elastic 

 vapour the atmosphere may contain ; and it is proba- 

 ble, although there are no experiments upon the sub- 

 ject, that it is increased in proportion to the rare- 

 faction of the air. 



72. Since a plant does not perspire at night, and 

 since its absorbing points, the roots, remain during 

 that period in contact with the same hamid medium 

 as during the day, they will attract fluid into the 

 system of the plant during the night, and, conse- 

 quently, the weight of the individual will be increased, 

 as Hales found to be the case. In like manner, 

 if plants in the shade are abundantly supplied with 

 moisture at the roots, they also will gain more than 

 they can lose ; and, as this will be a constant action, 

 the result must necessarily be to render all their 

 parts soft and watery. 



73. It is evident, J&om what has been stated, that 

 leaves must derive the food they digest from the 

 earth through the medium of the roots; and that 

 they, while alive, maintain a kind of perpetual suck- 

 ing action upon the stem, which is communicated to 

 the spongelets. That this must be of a very powerful 

 nature is apparent from the fact) that the smallest 



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