60 
POPULAR SCIENCE EEVIETT. 
in tobacco and beetroot, while copper has been frequently observed 
in vegetable products used for food, and, as it exists in tbe 
feathers of certain birds, the plantain-eaters of west and south 
Africa, it is most likely present in their vegetable food. 
Arguing, then, from such data as the above, we may classify 
the constituent elements of plants into those which are invariably 
and those which are exceptionally present. In our previous list 
are included 22 elements: of these therefore -the first 15 are 
general and invariable; the latter seven particular and excep- 
tional. Our knowledge on this point is, however, certainly open 
to modification, and in speaking of all plants we are not in a 
position to include under this term the lower and simple forms 
of vegetable life, of the ash constituents of which but little has 
yet been ascertained. 
Of the 15 invariable elements of plants to which our further 
remarks must be confined, six are metals and nine non-metals. 
The six metals are potassium, calcium, sodium, iron, magnesium, 
and manganese ; the eight non-metals are carbon, nitrogen, hy- 
drogen, oxygen, phosphorus, sulphur, silicon, chlorine, and fluorine. 
None of these elements, save oxygen and nitrogen, exist as such 
in the plant; they are not contained in it free, but combined. 
In what forms of combination they really exist we cannot here 
stop to inquire ; we confine ourselves to the question — In what 
forms are they supplied to and used by the plant ? 
Attempts to feed a plant upon its fourteen necessary elementary 
constituents in a pure and uncombined state would not succeed; 
it is not nourished by them, but perishes ; they are for the 
most part positively poisonous to it. Carbon, calcium, and 
the rest of these elements are not food to the plant, save when 
they exist in certain kinds of combinations. Pure, uncombined, 
they are no more food to the plant than they are to the animal. 
Dissected or analysed food, though it be elements of food, is 
not food ; it is as useless to the plant as a stone in lieu of bread 
to the animal. . The right elements indeed must be there, but 
they must be rightly combined. Not as free elements, but only 
after they have entered into combination with certain other 
elements, and that in certain proportions, are they taken up 
and assimilated by the plant. For these compounds, the 
ingredients of plant food, we have to look in three places — 
the air, the water, and the earth : the air which surrounds the 
stem and foliage of the plant, the soil in which its roots pene- 
trate, and the water which, in the form of rain, dew, and mist, 
comes in contact both with foliage and root. 
A plant, then, while building up by its own growth those 
organic materials which shall serve for the food of the animal, 
can make use of certain compounds only. The cycle of exist- 
ence in which the inorganic world lends its materials to the 
