504 
On the Food of Plants. 
these bodies seem to be essential constituents of the plant ; others 
are accidental and variable. We shall have to discuss these 
matters at length further on. 
The proximate vegetable principles may be divided into two 
great sections, namely, what may be called special products of 
certain particular plants, and sometimes of particular organs and 
general products of vegetable life, or those substances which are 
always present in every plant, and together make up the great 
bulk of the solid portion of its substance. To the first class 
belong the vegeto-alkalis, morphia, cinchonia, quina, &c. ; pe- 
culiar colouring matters, such as the red dye of madder, the body 
which on contact with air produces indigo, &c. These substances 
usually occur in very minute quantities, and although exceedingly 
interesting in many important relations, do not require extended 
notice in a paper of the present kind. 
Not so, however, with the second group, the members of which 
demand our close and attentive study if we wish to extend our 
knowledge of the phenomena of vegetable life. 
The bodies in question allow of easy and natural division into 
four classes, namely, neutral substances, vegetable acids, oils arul 
resins, and albuminous matters ; those last mentioned alone con- 
tain nitrogen. 
'The most important of the first class are sugar, starch, gum, 
and lignin, or woody fibre. The chemical history of these sub- 
stances is highly interesting ; the curious transformations they 
may be made to undergo even by the artificial processes of the 
laboratory, and the great importance of some of them as articles 
of food to man and animals, concur in rendering their study one 
of the most engaging portions of organic chemistry. The com- 
position of these principles is also very remarkable ; they all con- 
sist of carbon, hydrogen, and oxygen, the two latter being always 
in the proportion to form water — hydrates of carbon they have 
been called, but without a shadow of reason that such is their real 
constitution : they differ considerably in their physical characters, 
some being soluble, others not, some sapid, others tasteless. The 
name " indifferent,'' or " neutral," is given to them on account of 
the little tendency they possess to enter into chemical union with 
other substances, and is convenient, although not rigidly true, 
inasmuch as most of them can be made to combine with such 
bodies as lime and baryta and oxide of lead. 
Of these neutral principles, lignin, or woody fibre, and starch 
deserve the most attention ; the first by reason of its abundance in 
all plants, constituting as it does the great bulk of the solid 
woody matter, and the second on account of its extraordinary 
structure, which forms as it were a connecting link between 
strictly organised bodies and those which are crystallizable ; that 
