196 HOW CROPS GROW. 



grape. It doubtless occurs perhaps, together with Cae- 

 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, 

 Exp. Physioloyie, 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, JBaryta,a,nd. 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 algaB, is not known to exist 

 to any considerable extent or to be essential in any culti- 

 vated plants. 



Q 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 have found that these 

 bodies, in the absence of potash, act as poisons to the oat. (Vs. St., VIII, p. 147.) 

 t By some botanists ranked as a distinct species. 



