€4 Vegetable Physiology. 



bination, such an analysis of vegetable or animal substances 

 would miss their most essential characteristics. 



When we meet with a mass of mineral substance, such as a 

 block of marble, a ' nugget ' of gold, or the like, we know that 

 to ascertain the composition, it will suffice to scrape off a small 

 portion from any part of it and analyse this to acquire a tolerably 

 accurate knowledge of the whole. We can ' sample ' it by 

 fragments. It is far different when we have to deal with any 

 kind of living thing. We may analyse any plant in mass, and 

 thus ascertain its general chemical composition ; but if we take 

 separate portions of this same plant, we shall find the com- 

 position varying in every part ; that of the roots will differ 

 from that of the stem, which again will differ from the leaves, 

 flowers, seeds, &c., and the elements will, moreover, be found 

 to differ in each of them respectively at different seasons of 

 growth. 



This is, indeed, no more than we should expect, since plants 

 and animals are not fixed and permanent bodies like minerals, 

 subject only to change from the accidental influence, as it may 

 be called, of external agents ; they are objects which present 

 continual change so long as they retain the prime characteristic, 

 life. This change every one knows to be connected in almost 

 all cases with a flow of liquid matter through the mass, convey- 

 ing nutriment, removing useless matter, or otherwise importantly 

 contributing to interrupt and restore the equilibrium or equable 

 composition of the whole. 



But this is not all ; even the solid constituents present differ- 

 ences of composition, not only in the clearly distinct parts of 

 the individual objects, but even within limits which can only be 

 comprehended after a microscopic examination of the textures 

 composing the body ; and we may find widely different chemical 

 substances collected together, lying undisturbedly side by side 

 and maintaining their independence, in the most minute frag- 

 ment of vegetable or animal substance which our instruments- 

 enable us to isolate or to perceive. 



The reason of this is plain. Chemistry teaches us that all 

 matter, dead or living, is derived from some sixty primary 

 elements, that is, substances which, according to our present 

 knowledge, must be regarded as simple, because we cannot de- 

 compose them. Such are the metals, for instance ; but by far 

 the most important to the physiologist are those which con- 

 stitute the principal bulk of animal and vegetable substances, 

 "the three gaseous elements, oxygeiu hydrogen, and nitrogen, 

 and the more variable element carbon, which is most familiar 

 to us in the form of charcoal. Those who have studied what 



