On the Food of Plants. 
507 ' 
The most important of these arc the tartaric, citric, and malic. 
We know httle or nothing concerning the office performed by 
these acids. 
The composition of the three acids mentioned is such that they 
contain an excess of oxygen over that required to form water witii 
their hydrogen, and to this excess the character of acidity has 
been attributed. This can hardly be true, since two of the most 
powerful among organic acids, the acetic and the lactic, contain 
oxygen and hydrogen in the relation to form water. 
Oili/ and Resinous Principles. — Frequently found in special 
vessels, like enlarged cells, in particular parts of the plant. 
Sometimes they form a waxy varnish to the leaveS;, and sometimes 
occur in large quantities in the seeds. 
Oils are divided into Volatile, or those which distil over un- 
changed, and Fixed, or such as undergo partial decomposition 
when the attempt is made to convert them into vapour. The 
former have generally a powerful taste and smell, which is not 
the case with the latter. Some of the volatile oils, such as those 
of turpentine and lemon-peel, contain no oxygen ; they are car- 
burets of hydrogen, and present interesting cases of isomerism. 
Resins are looked upon ,as products of the gradual oxidation of 
volatile oils, with which they are always associated in nature. 
Altogether these jnatters are too purely chemical to require dis- 
cussion in the present case. 
Azotized Principles, Albuminous Matters. — These substances 
are exceedingly important, but it is only very lately that this 
importance has been placed in its proper light by the discovery 
made by the illustrious Liebig of the absolute identity of some at 
least of these bodies with the so-called proximate constituents of 
the animal frame, albumen, fibrin, and casein. This is no con- 
jecture; it is established by the most unequivocal chemical evi- 
dence: the origin of these substances is to be sought in the plant 
on which the animal feeds ; arrived in his stomach, they undergo 
simple solution in a peculiar manner, and are then directly ab- 
sorbed into his system. 
Albumen in an uncoagulated state is found in most vegetable 
juices : when these are boiled, the albumen becomes insoluble and 
separates. The substance called " gluten," which remains when 
dough of wheaten flour is kneaded in a stream of water, whereby 
the starch and soluble matters are washed away, and which forms 
so important a constituent of wheat as an article of food, consists 
chiefly of a mixture of two substances, differing in properties, but 
having the same composition, one of which is identical with the 
fibrin of the blood. The other has not been thoroughly examined. 
Beans, and many other seeds which contain oil, such as almonds, 
besides albumen, are loaded with a substance quite indistinguish- 
