126 STARCH CONVERTED INTO WOODY FIBRE. 



§ 10. General observations on the substances of which plants chiefly consist. 



It may be useful here shortly to review the most important facts and 

 conclusions which have been adverted to in the present lecture. 



1°. The great bulk of plants consists of a series of substances capable 

 of being represented by, and consequently of being formed in nature 

 from, carbon and the elements of water only. Such are woody fibre, 

 starch, gum, and the several varieties of sugar (p. 111). 



2°. Yet the crude mass of wood, as it exists in a full-grown 

 tree, containing various substances in its pores, cannot be represented 

 by carbon and the elements of water alone. It appears always to 

 contain a small excess of hydrogen, which is greater in some trees than 

 in others. Thus in the chesnut and the lime, this excess is greater than 

 in the pines, while in the latter it is greater than in the oak and the ash. 

 [For a series of analyses of different kinds of wood by Peterson and 

 Schodler, see Thomson's Organic Chemistry, p. 849.] 



3°. These substances are, in many cases, mutually convertible even 

 in our hands. They are probably, therefore, still more so in nature. 



It is to be observed, however, that all the transformations we can as 

 yet effect are in one direction only. We can produce the above com- 

 pounds from each other in the order of lignin or starch, gum, cane sugar, 

 grape sugar — that is, we can convert starch into gum, and gum into 

 sugar, but we cannot reverse the process, so as to form cane from grape 

 sugar, or starch from gum. 



The only apparent exception to this statement with which we are at 

 present acquainted, occurs in the case of starch. When this substance 

 is dissolved in cold concentrated nitric acid, and then mixed largely with 

 water, a substance [the Xyloidin of Braconnot] falls to the bottom, 

 which is a compound of the nitric acid with woody fibre (C,2 Hg Og.) 

 [Pelouze, see Berzelius Arsberdttelse, 1839, p. 416.] In this instance, 

 if the above observation is correct, there appears to be an actual con- 

 version.of starch into woody fibre. 



But what we are as yet unable to perform may, nevertheless, be easily 

 and constantly effected in the living plant. Not only may what is starch 

 in one part of the tree be transformed and conveyed to another part in 

 the form of sugar, — but that which, in the form of sugar or gum, passes 

 upwards or downwards with the circulating sap, may, by the instrumen- 

 tality of the vital processes, be deposited in the stem in the form of 

 wood, or in the ear in that of starch. Indeed we know that such actu- 

 ally does take place, and that we are still, therefore, very far from being 

 able to imitate nature in her power of transforming even this one group 

 of substances only. 



4°. Among, or in connection with, the great masses of vegetable mat- 

 ter which consist mainly of the above substances, we have had occasion 

 to notice a few which contain nitrogen as one of their constituents — and 

 which, though forming only a small fraction of the products of vegetable 

 growth, yet appear to exercise a most important influence in the general 

 economy of animal as well as vegetable life. The functions performed 

 by diastase in reference to vegetable growth, and to the transformations 

 of organized vegetable substances, have already been in some measure 

 illustrated, — we shall hereafter have an opportunity of considering more 



