124 LECTUBES ON 



Grape sugar is tbus seen to be the final product of the transfor- 

 mation of the carbo-hydrates, either in the vessels of the chemist or 

 in the digestive process of animals. It is the form in which the 

 carbo-hydrates of the food pass into the blood, and, in consequence, 

 it is a constant ingredient of the latter. 



It will be noticed that while physical and chemical agencies pro- 

 duce these metamorphoses in one direction, it is only with the assist- 

 ance of the vital principle that they can be accomplished in the 

 reverse manner. 



In the laboratory we can only reduce from a higher, organized, or 

 more complex constitution, to a lower and simpler one. In the vege- 

 table cell, however, all these changes, and many more, take place 

 with the greatest facility. 



The ready convertibility of one member of this group into another 

 is to some extent explained by the identical or similar composition of 

 these bodies. It will be observed by reference to the table that they 

 are all composed of carbon, hydrogen, and oxygen. 



That they contain carbon is made evident by their yielding charcoal 

 when heated with imperfect access of air. When heated they also 

 yield water, which, as all know, is a compound of oxygen and hydro- 

 gen. 



Furthermore, several of these bodies contain the same proportions of 

 these elements. The formulae of cellulose, starch, inulin, and dextrin 

 are identical. The remaining compounds only differ by the elements 

 of one or several atoms of water. 



The term carbo-hydrates (very convenient for our present purpose, 

 though to the chemist absurd) was applied here because we may in 

 a certain sense consider all these substances as hydrates of carbon. 

 They are, in fact, composed of carbon and the elements of water. 



These bodies in their transformations have merely to undergo a 

 rearrangement of atoms, just as the rearrangement of a few blocks 

 enables the child to build a variety of toy-houses; or at most they 

 need only lose or assume a few atoms of water — an omnipresent body, 

 characterized by the facility with which it enters into all manner of 

 combinations — and the work is accomplished. 



To furnish a more complete illustration of the typifying of all vege- 

 tation by the single cell, and at the same time to extend our inquiry 

 into the composition of the plant, we may now advert to the lining 

 membrane of the cell-wall, or, as physiologists term it, the pro- 

 toplasm, formative-layer, or primordial utricle. This consists 

 chiefly of some body that differs in chemical composition from the 

 group just described by containing, in addition to the three elements 

 that form the carbo-hydrates, about 16 per cent, of a fourth element, 

 nitrogen, and small quantities of sulphur, and perhaps sometimes 

 phosphorus. 



The following table gives the names and percentage composition of 

 the most important. 



