196 HOW CROPS GROW. 
grape, It doubtless occurs perhaps, together with Cas. 
sium, in many other plants, though in very minute quan- 
tity. It is not unlikely that small quantities of these 
alkali-metals may be found to be of decided influence on 
the growth of plants.* 
The late investigations of A. Braun and of Risse, (Sachs, 
Fixp. Physiologie, 153,) show that Zinc is a usual ingre- 
dient of plants growing about zine mines, where the soil 
contains carbonate or silicate of this metal. Certain mark- 
ed varieties of plants are peculiar to, and appear to have 
been produced by, such soils, viz.,a violet, ( Viola tricolor, 
var. calaminaris,)+ and a shepherd’s purse, (Zhlaspi al- 
pestre, var. calaminaris.) In the ash of the leaves of the 
latter plant, Risse found 13°|, of oxide of zinc; in other 
plants he found from 0.3 to 3,3" |,. 
Copper is often or commonly found in the ashes of 
plants; and other elements, viz., Arsenic, Baryta,and Lead, 
have been discovered therein, but as yet we are not fairly 
warranted in assuming that any of these substances are of 
importance to agricultural vegetation. The same is true 
of Zodine, which, though an invariable and probably a 
necessary constituent of many alge, is not known to exist 
to any considerable extent or to be essential in any culti- 
vated plants. 
§ 4. 
FUNCTIONS OF THE ASH-INGREDIENTS. 
But little is certainly known with reference to the 
subject of this section. 
Sulphates.—The albuminoids, which contain sulphur as 
an essential ingredient, obviously cannot be produced in 
absence of sulphuric acid, which, so far as we know, is the 
®* Since the above was written, Birner & Lucanus lave found that these 
bodies, én the absence of potash, act as poisons to the oat. (Vs. St., VIIL p. 147%." 
+ By some botanists ranked as a distinct species. 
