196 HOW CROPS GROW. 



grape. It doubtless occurs perhaps, together with 6W 

 ftium, 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&quot; decided influence on 

 the growth of plants.* 



The late investigations of A. Braun and of Risse, (Sachs, 

 Exp. Pliysiologie, 153,) show that Zinc is a usual ingre 

 dient of plants growing about zinc 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, (Thlaspi 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^^. 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 Iodine, which, though an invariable and probably a 

 necessary constituent of many alga3, is not known to exist 

 to any considerable extent or to be essential in any culti 

 vated plants. 



.1*. -; I 



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 & Lncanus lave found that these 

 bodies, in the absence of potash, act as poisons to the oat. { Vs &amp;lt;S2., VIII, p. 147.J 

 t By some botanists ranked as a distinct species. 



