210 HOW CROPS GROW. 



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

 Exp. Physiologic, 153) show that Zinc is a usual ingredi- 

 ent of plants growing about zinc-mines, where the soil 

 contains carbonate or silicate of this metal. Certain 

 marked 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 alpestre, 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%. These 

 plants, however, grow equally well in absence of zinc, 

 which may slightly modify their appearance, but is unes- 

 sential to their nutrition. 



Boron as boric acid has recently been found in many 

 wines of California and Europe. 



Copper is often or commonly found in the ashes of 

 plants ; and other elements, viz., Arsenic, Barium and 

 Lead, have been discovered therein, but as yet we are not 

 warranted in assuming that any of these substances are 

 of importance to agricultural vegetation. The soluble 

 compounds of copper, arsenic and lead are in fact very 

 injurious to plant life, unless very highly diluted. 



Iodine, an invariable and probably a necessary constit- 

 uent of many algae, is not known to exist to any consid- 

 erable extent or to be essential in any cultivated plants. 



8 4. 



FUNCTIONS OP THE ASH-INGREDIEXTS. 



Although much has been written, 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 sulphates, which, so far as we know, are the 

 exclusive source of sulphur to plants. The sulphurized 



