CHAPTER VII. 



OF THE STRUCTURE OF LEAVES. 



214. The fluid which is so abundantly taken up by the Roots 

 of plants, and which is conveyed upwards along the interior of 

 the Stem, is very unfit for the nourishment of the structure, and 

 for the supply of the growing parts, until it has been exposed to 

 the influence of the air, by which great changes are effected in 

 its properties. Now this object has to be attained in Animals, 

 as well as in Plants ; and we observe two modes of effecting it. 

 In some Animals, the blood is sent into very delicate external 

 prolongations of the skin, termed gills ; through the thin mem- 

 brane c"f which, it may receive the required influence. And, 

 although we usually see an apparatus of this kind, existing only 

 in animals which inhabit the water (the air diffused through 

 which is really that which acts on the blood), yet it is seen in 

 some air-breathing animals also. The usual mode, however, in 

 which the blood is exposed to the influence of the atmosphere, in 

 animals living on land, is by the introduction of air into cavities 

 termed lungs within the body, constituting the process known as 

 respiration or Ireathing ; but this requires a series of movements, 

 for the constant exchange of the air so introduced ; in order that 

 the portions rendered unfit for further use, by the changes that 

 take place in it, may be expelled, and a fresh supply admitted. 

 A little consideration will show, that to have introduced water 

 in a similar manner, into the interior of the bodies of those ani- 

 mals which inhabit it, would require an immense amount of 

 force, since water is so much less easily moved than air ; whilst, 

 on the other hand, to have furnished air-breathing animals with 

 external gills or other similar appendages, would have exposed 



