THE FOOD OF PLANTS. 
59 
Of the 65 l^nown elements, 13 are universally allowed to be 
absolutely necessary constituents of all plants. To these may be 
added two others, fluorine and manganese, which are probably 
present in minute proportions in all plants ; although it is only 
of late years that we have acquired distinct evidence on this point. 
The reasons for suspecting fluorine and manganese to exist in 
plants may be here cited as excellent illustrations of the indi- 
rect way in which obscure constituents of plants have been dis- 
covered. Fluorine, it is well known, exists in the bones and 
teeth of the higher animals in notable proportion, in the form of 
calcic fluoride. Now, if the plants on which such animals chiefly 
feed did not contain fluorine, whence could they draw constant 
supplies of this necessary element? Not by chance traces ot 
fluorides in the particles of earth taken into their system along 
with the herbage ; this plan would be not only contrary to 
analogy, but too capricious to be relied on. Now, very careful 
analyses of many plants have shown them to contain a distinct 
proportion of fluorine. The phosphoric acid which forms so large 
a part of bone is not more certainly contained in vegetables and 
supplied by them to animals than is fluorine. Moreover, it is 
instructive to note that both phosphoric acid and fluorine are 
present together even in such minerals as apatite, osteolite, &c., 
just as they are also together present in bones. With regard 
to manganese, it constantly accompanies iron in the mineral king- 
dom ; and in plants likewise which contain much iron a con- 
siderable proportion of manganese is often found. One way in 
which the existence of manganese in a plant may be anticipated 
is of considerable interest. The ash of silk contains manganese. 
Even if one fails to detect this element in the ash of the egg, one 
1ms no difficulty in finding it in the ash of a single cocoon. 
Whence has this metal been derived then ? Of necessity from 
the mulberry leaves, or other vegetable, on which the silkworm 
has been fed. But if manganese and fluorine be accepted as 
general and essential elements of all plants, there are other con- 
stituents the presence of which has not yet been shown to' be 
general. We have placed a star(^) opposite to the names of these 
elements. They have been found in particular orders, genera, or 
species of plants, but the history of their occurrence and distri- 
bution is still in some cases obscure. Aluminum has been found 
in certain species of Lycopodium and Selaginella ; the history of 
the Zinc violet, Viola calaminaria, of the neighbourhood of Aix- 
la-Chapelle, has been the theme of much interesting discussion. 
Bromine and iodine are found in marine algae, and, indeed, the 
latter of these elements might still have remained unrecognised, 
had not it been accumulated in sea-weed from the minute pro- 
portion of it existing in the ocean. The rare and recently-dis- 
covered alkaline metals, caesium and rubidium, have been detected 
