172 INSECTICIDES, FUNCrlCIDKS, AND WEED KILLERS. 



they were not able to find ponderable quantities in the organs cured. 

 Of a contrary opinion Dufour describes the absorption of these salts 

 by the surface of the leaf, and Weiss and Wiesner have shown that iron 

 winch enters into the plant exists there in organic combination which 

 may have escaped Millardet and Knop. Frank, Kruger, and Vedrosi 

 regard green vitriol as equal to blue vitriol as a stimulant of the plant, 

 and believe that a very small dose suffices to produce a visible and 

 salutary effect. These two salts seem to act in quite the same manner 

 on plants with this difference, that copper salts possess an action ten 

 times stronger than iron salts taking into account their different 

 chemical equivalent. Shrubs which annually vmdergo the classical 

 treatment with bouillie bordelaise, likewise those which are treated with 

 solutions of green vitriol, acquire a greatly increased vitality which in- 

 creases assimilation and a longer vitality which enables trees to preserve 

 their leaves in autumn much longer than those untreated. These facts 

 prove that iron is the active chemical agent in the formation of 

 chlorophyll, stimulating its functions, and in that way also it becomes 

 the indirecD provider of starch, sugar, and cellulose for the plant. 

 Without iron, on the other hand, the plant eventually dies because 

 assimilation of carbonic acid iDecomes impossible. The role of iron there- 

 fore seems analogous to that of potassium, the absence of the latter 

 element also inducing the aniemia of plants. The substitution of iron 

 for potassium according to Griffiths' experiments would be very ad- 

 vantageous in the struggle against the cryptogamic diseases of plants. 

 Potash is very favourable to the development of fungi, whilst green vitriol 

 is injurious. The substitution of green vitriol for potassic manures 

 whilst giving the same physiological results would have the effect of 

 circumscribing the cryptogamic diseases of plants. Desjardin had al- 

 ready observed the great resistance of plants treated with green vitriol 

 to disease. Chavie-Leroy certifies that the simultaneous use of green 

 vitriol and gypsum preserves grain crops from rust and laying; it 

 diminishes ergot, the fall of the flower of the vine, stops the canker of 

 apple and pear trees, and causes the gum of stone-fruit trees to dis- 

 appear. Like gypsum, green vitriol used in the soil acts on the latter 

 from a physical and chemical point of view. Boussingault agrees with 

 Sachs, Stohmann and Knop (1) that it oxidizes the organic matter of 

 humus and hastens its decomposition ; (2) that it fixes ammonia in the 

 soil ; (3) that it aids the plant to absorb the phosphoric acid in the 

 soil. Thenard and Joulie state that phosphate of lime is converted 

 in the soil, in contact with oxide of iron, into phosphate of protoxide of 

 iron soluble in water charged with carbonic acid ; the ferric phosphate 

 formed would afterwards be reduced by the organic matter. In 1859 

 Knop was already convinced of the action of iron as a vehicle for 

 phosphoric acid. Like gypsum, green vitriol effects a radical change 

 in the soil by the decomposition of the insoluble minerals which con- 

 tain potash. Green vitriol forms an equivalent of a soluble potash 

 salt and thus renders the richness of the soil available for the plant. 

 It follows that green vitriol having always a beneficial action on crops 

 is thus capable of increasing the yield and that chiefly a^ i^egards 

 plants to which potash is necessary. Where carbonate of lime is the 



