II Sept., 1911] Absorption of Food Substances through Leaves. 619 



THE ABSORPTION OF FOOD SUBSTANCES AND 

 POISONS THROrCIH LEAVES. 



Alfred J. Euart. D.Sc., PIt.D., F.L.S.. Government Botanist and- 

 Professor of Botan v and Plant PJiysioIagy in the Mclliournc University. 



It has hitherto been generally accepted in Plant Physiology that the 

 leaves of flowering plants are organs specially adapted for carrying out 

 gaseous exchanges with the atmosphere, and that they have comparatively 

 little power of absorbing either water or solids which may be dissolved in 

 water wetting the surface of the leaf. In the case of such plants as 

 mosses, lichens and in the leaves of carnivorous plants, however, a very 

 pronounced power of absorbing water and dis.solved solids is shown. The 

 same, of cour.se, also applies to the leaves of water plants which grow^ 

 completely submerged in water. In fact, whether a leaf will be able to 

 absorb water and dis.solved solids, or not, depends mainly upon the thick- 

 ness and character of the outermost layer or skin, known to botanists as 

 the cuticle. This varies greatly in its thickness and impermeability to 

 water, according to the kind of leaf or the conditions under which it has 

 grown ; but. even in the ca.se of a leaf like that of the beech which has a 

 fairly well developed cuticle, a simple experiment suffices to show that 

 solids can be absorbed directly through the outer surface of leaf. For 

 instance, if a drop of a ver^^ dilute solution of potassium nitrate be placed 

 upon a beech leaf and covered with a small bell-jar so as to check e\'apora- 

 tion, when the drop finally disappears no crystals are left behind on the 

 surface of the leaf ; wiiereas, if the drop is allowed to evaporate rapidly, 

 a little crystalline efflorescence is left behind, owing to the fact that the 

 salt had not time to be absorbed before the water had evaporated. Of 

 course, in a condition of Nature, rain water usual Iv does not contain 

 more than traces of dissolved salts and usually does not remain long in 

 contact with the leaf. Nevertheless, some recent investigations pul)li.shed 

 in the Mitteilungen der PeutscJien Landwirtschafts GeseUschaft. 191 1, 

 page 231. by Profe.s.sor Hiltiier. Professor (if Agriiulture at the University 

 of Munich, .seem to show that quite appreciable quantities of lv>th food 

 substances and poisons ma\ be absorbed by the lea\es when such substances 

 are sprayed on the foliage, either dissolved in water or in suspended form. 



.Some instances of this power of absorption through the leaves, have 

 long been kncnvn. For instance, if the leaves of a plant happen to have 

 become y)ale coloured, as .sometimes (Xcurs owing to a deficiency of iron 

 in the .soil or to a difficulty in absorbing it, then merely painting the pale 

 surface of the leaf with a dilut<' solution of chloride of iron will restore the 

 green colour temporarilv or perniancntlv to the leaves so treated. Another 

 instance, in this ca.se of the absorption of a poison, is the well known use 

 of copper sulphate to ket]i down ('harhak or Wild Mustard in ct-real 

 crops. The copper sulphate .solution adhi-res long enough to the Charlock 

 leaves to W' ab.s()rl)ed by them and to cau.se tln*ir dtath. whereas the solu- 

 tion runs off the grass leaves without aflecting them to any appreciable 

 extent. Professf)r Hiltner's e\|)erinients seem mainly to have been directed 

 towards det<Tniining whether it would be possible bv spraying potassium 

 salts oil the leaves to obtain a more rapid respt>n.se than if the salts were 

 applied to the soil, ('ritical <^xperini<-nts were performed with mustard 

 and .soy beans fr<>m wirKJi it ai)]ieared that neitluM" nitrogenous t<X)d 



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