CHEMICAL CONTROL OF DIPPING TANKS. 153 



4. Water containing an appreciable amount of saline matter 

 (especially salts of iron and of the alkaline earths), when used in 

 making up a dip fluid, may cause a serious precipitation of both 

 the oxidised and the unoxidised arsenic in the dipping tank. Salts 

 of sodium and potassium in the water would have no such dele- 

 terious effect. 



A PRELIMINARY ACCOUNT OF SOME INVESTIGATIONS 

 ON LEAF-AERATION IN CERTAIN NATAL PLANTS. 



By G. W. Gale, B.Sc, 



Union Government Research Scholar. 



Read July 11, 1921. 



The method of estimating the intercellular space content of 

 Angiosperm leaves by noting their increase in weight when 

 injected (under reduced pressure) with water, dates back to 1721, 

 when Christian Wolff first made experiments of this sort. In 

 1854, Unger made an extensive series of such measurements on 

 various species chosen apparently more or less at random, and 

 helped to pave the way for Haberlandt's generalization in 1914 

 that the aeration system of leaves is greatest under hygrophilous 

 conditions and least under xerophytic conditions. As a corollary 

 to this statement we know that shade leaves exhibit a greater 

 degree of aeration than sun leaves, a conclusion confirmed by 

 McLean's work on the tropical rain-forest of Brazil. 



Last year the present writer commenced work on the aeration 

 systems of leaves with the object of comparing, in this respect, 

 different ecological types of the Natal flora. A brief description 

 of the method used, and of certain necessary precautions, may be 

 of interest. The leaves are weighed and placed in a conical flask 

 attached by a side-exit to an air-pump; the injection-fluid enters 

 from a reservoir placed above the flask and communicating with it 

 by means of a tube controlled by a stopcock. The air is exhausted 

 from the flask, and this of course withdraws also the air from the 

 intercellular spaces. The stopcock control is now opened and fluid 

 run in from the reservoir until the leaves are completely covered. 

 A few more turns of the air-pump are now necessary in order to 

 exhaust the air dissolved in the injection fluid. The flask is now 

 disconnected from the pump, and as the air rushes in the leaves 

 are injected. They are then taken out, dried with a linen cloth, 

 and re-weighed. Acting on McLean's suggestion, 4% alcohol 

 (sp. gr. -99) is used as the injection fluid; it is too weak to cause 

 plasmolysis of the cells, but possesses a much lower surface tension 

 than water and hence penetrates more easily. The process of 

 injection can actually be observed in nearly all leaves, and thus, 

 there is a guarantee that injection shall always be complete. 



