Development of Roots of Agricultural Plants. 427 '* 
come into intimate contact with the soil, and penetrate through 
any crevice, however narrow, in a rock or drain-pipe in search 
of nutriment. 
We are not to suppose that fluid enters into the spongelet by 
mere endosmose,* though from the strong demands for fresh 
supplies constantly made by evaporation from the leaves, the 
admission is comparatively easy, without any active exosmose. 
This agency would be, perhaps, sufficient to account for the rapid 
admission of fluid after a season of extreme drought ; but in dry 
weather, when there is little moisture in the soil, or in extremely 
wet weather when there is little or no evaporation, vital energy 
must be taken into account in order to have some notion of what 
takes place. 
The most important question, perhaps, which arises in con- 
sidering the relation of the roots to agriculture, is what power 
they have in the selection of food. That they have some is 
apparent from the fact that different plants in the same soil will 
appropriate different proportions of chemical matters, and that 
some exhaust more rapidly than others particular constituents. 
Wheat and peas appropriate very different proportions of silica ; 
Tamarisk and Salsola in the same ground give very different 
chemical results, the former containing more magnesia and the 
latter more soda ; while in Eryngiiim maritimum, which grows 
exclusively on the sand which borders the sea, and is strongly 
impregnated with chlorate of soda, there occurs three times as 
much potash as soda. Lycopodia, with some other cryptogams, 
are remarkable amongst plants for largely appropriating alumina. 
But more than this, some plants have the property of appropriating 
enormous quantities of some especial element, when it exists in 
infinitesimal quantities in the surrounding medium. The quantity 
of iodine in sea-water is so small as scarcely to be detected 
except from the highly concentrated fluid, and yet several marine 
plants contain great quantities. "The ash of Viola calaminariOy 
according to Alexander Braun, a plant which, in the parts about 
Aix-la-Chapelle, is held so strongly indicative of the presence of 
zinc, that the places where it grows are selected for opening new 
mines in search of zinc-ore, is found to contain oxide of zinc," a. 
substance which few plants could appropriate.! Great, indeed, 
as are the powers of vitality in effecting chemical change, we have 
* The words endosmose and exosmose are used to express the mutual actioa 
between two fluids of different specific gravities on different sides of a thin 
membrane, by means of which the one enters (by endosmose) and the other exudes- 
(by exosmose). Gaseous bodies enter and are given out in like manner ; and 
when different gases are mixed together, they have different rates of penetration,, 
one being admitted more rapidly than another. A familiar instance is afforded 
by the process of oxygenating and decarbonizing the blood in animals. 
t Liebig, • Natural Laws of Husbandry,' p. 57. 
