288 PERSPIRATION. 



the human surface. Generally the sap loses about two thirds of its water, and the 

 exhaled portion is probably pure, or does not contain more than a 10,000,000th 

 part of the foreign matter which it had when first absorbed.^ (Dr. Roget, Bridge- 

 water Treatise, vol. ii. p. 27. sq.) 



The greater number of cellular plants absorb water equally at every part of 

 their surface. Lichens only in particular parts. In vesicular plants the surface 

 absorbs but little, except when the roots have been removed, or can obtain no 

 water. 



