LECTURE VI. 



abstances of which plants chiefly consist— Woody fibre, Starch, Gum, Sugars— Their mu- 

 tual relations and transformations- Gluten,Vegetable Albumen, Diastase— Acetic, Tartaric, 

 Malic, Citric, and Oxalic Acids — General observations. 



From what has been stated regarding the structure of plants, it will be 

 u.nderstood in what way the food is introduced into their circulation. The 

 rext inquiry appears to be how — by what chemical changes — is the food, 

 when introduced, converted into those substances of which plants chiefly 

 consist. But in order that we may clearly understand this point, it is 

 necessary that we know first the nature and chemical constitution of the 

 substances which are most largely formed from the food in the interior 

 of the plant. To this point, therefore, I must previously direct your 

 attention. 



If you were to collect all tfie varieties of plants which are within your 

 reach — whether such as are cultivated and used for food — or such as 

 grow more or less abundantly in a wild state — and were to extract their 

 several juices, and to separate from each of these juices the chemical 

 compounds it contains — you would gradually gather together so many 

 different substances, all possessed of different properties, that you would 

 scarcely be able to number them. 



But if at the same time you compared the weight of each substance 

 thus collected with that of the entire plant from which it is derived, you 

 would find also that the quantity of many of them is comparatively so 

 minute that only a very small portion of the vital energies of the plant 

 can be expended in producing them, — that they may be entirely neglect- 

 ed in a general consideration of the great products of vegetation. Thus 

 though quinine and morphine, the active ingredients in. Peruvian bark 

 and in opium, are most interesting substances, from their effect upon the 

 human constitution, and their use in medicine, yet they form so small a 

 fraction of the mass of the entire trees or plants from which they are ex- 

 tracted, that it would be idle to attempt to convey to you any notion of 

 the way in which plants grow and are fed, by showing you how such 

 substances as these are produced from the food on which plants live. 



While, however, the examination would satisfy you that almost 

 every species of plant produced in small quantity one or more sub- 

 stances peculiar to itself, you would observe, at the same tiine, that 

 every plant yielded a certain quantity of two or three substances com- 

 mon to and produced by all, and in most cases constituting the greater 

 portion of their bulk. Thus all trees and herbs produce wood or woody 

 fibre, and of this substance you know that their chief bulk consists. 

 Again, all the grains and roots you cultivate contain starch in large 

 quantity, and the production of this starch is one of the great objects of 

 the art of culture. The juices of trees, and of grasses, and of cultivated 

 roots, contain sugar and gum, and sometimes in such quantity as to 

 make their extraction a source of profit both to the grower and to the 



