THE FOOD OF PLANTS. 73 



ammonia is not oxidised in the plant to Nitric acid, before being 

 utilised for the formation of proteids. But still, the experiment 

 teaches that ammonia can be employed by the higher plants as 

 food. 1 



1 See G. Kiihn and Hampe, Versucfisstationen, 1867. 



24. The Seat of Proteid Formation in the Higher Plants. 



In the leaves large quantities of carbohydrates are produced in 

 assimilation. The transpiration current conveys to the leaves 

 nitrates, and it may hence be assumed that the formation of 

 proteids goes on chiefly in the leaves. 



We prepare transverse sections through a mature leaflet of 

 Trifolium pratense (see Fig. 3). Within the epidermis we see the 

 single-layered palisade parenchyma, and the many-layered spongy 

 parenchyma. The bast and wood of the vascular bundles are 

 easily recognisable. The soft bast is enclosed on the outside by 

 bast fibres, and the wood is similarly bounded on the outside by 

 strongly thickened sclerenchymatous fibres. Outside the layer of 

 bast fibres, and also outside the thick- walled cells encircling 

 the wood, is a single layer of cells, devoid ,of chlorophyll, which 

 contain crystals of Calcium oxalate. The crystal sheaths only 

 cover the backs of the strands of fibres ; they do not extend far 

 laterally. 1 



Now since, as we shall see in 25, salts of Nitric acid (and also 

 of Sulphuric acid) can be decomposed under the conditions obtain- 

 ing in plants by the Oxalic acid which is so generally found in 

 plant cells, it is not difficult to suppose that the crystals of Calcium 

 oxalate referred to are to be regarded as the product of such 

 reactions. The Nitr*ic acid and Sulphuric acid can then, with 

 non-nitrogenous organic material formed in the green cells, be 

 employed under the influence of the protoplasm for the formation 

 of proteids. With reference to these questions, the following 

 observations and experiments claim particular attention. 2 



To prove the presence of Calcium oxalate in leaves (according 

 to my experience it is very convenient, in order to learn the 

 method, to use moderately young leaves of Humulus), we first 

 kill the specimens under investigation by dipping in hot water, 

 then extract them with alcohol, lay them in chloral hydrate 

 solution (5 parts of water to 8 parts of chloral hydrate), and 

 observe under the polarisation microscope. With crossed Nicols 



