In comparison with copper little work has been done with regard to the action of soluble zinc salts alone on higher plants when grown in water cultures. Freytag (1868) stated that zinc salts must be very dilute if the plants are not to be harmed, and that for zinc sulphate the concentrations must not be more than 200 mg. per litre (= 1/5000). Baumann (1885) carried out further experiments and concluded that zinc salts are far more toxic than Freytag suspected, 44 mg. zinc sulphate per litre[5] killing plants of 13 species belonging to 7 families (Coniferae excepted). The various plants withstand the action of the zinc salts in different degrees, the same concentration killing off the species in different times. With the 44 mg. zinc sulphate the following results were obtained:-- Trifolium pratense killed in 16 days Spergula arvensis „ 21 „ Hordeum vulgare „ 30 „ Vicia sativa „ 31 „ Polygonum Fagopyrum „ 60 „ Beta vulgaris „ 76 „ Onobrychis sativa „ 194 „ [5] 44 mg. ZnSO₄.7H₂O = 10 mg. Zn = 1/22,727 ZnSO₄.7H₂O approx. With still less poison, 22 mg. zinc sulphate per litre, all the species mentioned were eventually killed with the exception of _Onobrychis sativa_, while 4·4 mg. zinc sulphate seemed to be harmless for all the plants tested except _Raphanus sativus_, which is evidently exceptionally sensitive to this toxic substance. Jensen (1907) again indicated the poisonous action of zinc salts and also found that a relatively small reduction of toxicity was obtained by the addition of finely divided quartz to the solutions. (_b_) _Effect of soluble zinc salts in the presence of nutrients._ Krauch (1882) grew various plants in the presence of nutrient solutions and quantities of zinc sulphate varying from ·1 to ·8 gm. per litre (= 1/10,000 to 8/10,000). Barley proved to be very sensitive, even to the weakest strength of the poison, as the plants soon showed reddish flecks, while all were dead within six weeks, the control plants without zinc remaining quite healthy. Certain grasses took longer to kill than barley, those with ·4 gm. zinc sulphate per litre dying in about seven weeks, while 13 weeks elapsed before the others were killed. Even after this length of time the plants with ·1 gm. zinc sulphate per litre still survived, although in a very sickly condition. With willow, again, even ·1 gm. zinc sulphate per litre made the plants very sickly after four weeks, growth being weak, the leaves yellow, and the roots brownish. In this case the solutions were renewed, but the plants treated with zinc compounds were dead within eight weeks from the start, the controls being very healthy.