74 PHYSIOLOGY OF NUTRITION. 



the crystals of Calcium oxalate appear glistening white or coloured 

 on a black ground ; with parallel Nicols they appear almost black 

 on a white ground. The crystals occur in the cells in very various 

 forms. They belong partly to the tetragonal, and partly to the 

 monoclinic system. 3 If we lay the objects in concentrated Hydro- 

 chloric acid, and again observe under the polarising microscope, 

 we can easily make out the gradual solution of the oxalate. 

 Water and Acetic acid do not dissolve the crystals. 



Respecting the polarising microscope, consult Section II. 



If we examine by the above method on the one hand young 

 leaves, and on the other old leaves of Ampelopsis, we find in the 

 parenchyma numerous bundles of raphides, and, in fact, in about 

 equal quantities in both cases. Most Dicotyledons behave quite 

 differently, and do not carry the Calcium oxalate in their leaves 

 in the form of raphides. We examine e. g. young, just matured, 

 and older leaves of Alnus glutinosa, Ulmus campestris, and 

 Humulus. In all cases we take the leaves from shoots growing in 

 the shade, and find that the quantity of oxalate in the parenchyma 

 increases considerably with age. 



If a vigorous pot plant of Pelargonium is put in a badly illumin- 

 ated place, so that no assimilation can go on, then we find but 

 little Calcium oxalate in the parenchyma of the newly originating 

 leaves. The formation of these small quantities of oxalate is not 

 associated with the a-ction of light at all, and so we have here to 

 do with primary Calcium oxalate. If the leaves develop in the 

 dark, all formation of Calcium oxalate ceases. It begins again, 

 however, if the plant is brought into the light. Secondary 

 Calcium oxalate originates, whose production, unlike that of the 

 primary oxalate, is dependent on the light. (Schimper.) 



The material for the formation of the primary Calcium oxalate 

 is afforded on the one hand by the Oxalic acid produced in the 

 plant by metabolic processes, and on the other, by the lime 

 absorbed by the roots from without. The latter enters the plant 

 from the soil, chiefly in combination with Nitric acid and Sul- 

 phuric acid. If, now, the nitrates, for example, enter into 

 chemical reaction with the Oxalic acid produced in metabolism, 

 then, on the one hand, free Nitric acid is formed, which finds 

 employment in proteid manufacture, and on the other hand, 

 Calcium oxalate originates. Nitrates may also, however, tran- 

 sitorily accumulate in the plant tissue, as was shoAvn by the 

 observations mentioned in 21. 



