334 



PHYSIOLOGY OF NUTRITION. 



acid in certain tissues will be easily made out (particularly in the 

 cortex, bast parenchyma, and the usually unilayered medullary 

 rays). The contents of the cells containing Tannic acid are 

 coloured reddish brown. 



On examining longitudinal sections from the pith of a current 

 year's rose shoot, we see that it consists on the one hand of large 

 <5ells, and on the other of longitudinally running rows of cells 

 communicating with one another, which traverse the large-celled 

 tissue. If we examine sections from the pith, mounted in a drop 

 of a 10 per cent, aqueous solution of Potassium bichromate, we 

 find that the contents of most of the narrow cells are stained 

 reddish-brown. We can also detect the Tannic acid in the narrow 

 <5ells by placing sections from the pith in a drop of aqueous Ferric 

 chloride solution, or in a drop of Ferric sulphate solution. The 

 contents of the cells containing Tannic acid then stain dark blue. 

 In roses the leaves also are very rich in Tannic acid, and to 

 demonstrate this, e.g. in lecture, we fold up a leaf and crush it 

 with the finger on white blotting-paper, so as to squeeze out some 

 of the cell-sap. On touching the moistened parts of the blotting- 

 paper Avith a solution of Ferric chloride, the Tannic acid reaction 

 at once appears. 



Recently several attempts have been made to investigate 

 quantitatively the occurrence of tannins in plants, in order to 

 obtain a clearer understanding of the processes which bring about 

 the production of tannins in the organism. In these investiga- 

 tions, and also in microchemical studies of the genetic relation- 

 ships of tannins, it must not be forgotten that the substances 

 designated tannins may be of very different 

 chemical constitution, and hence all work in 

 this direction must still be of a provisional 

 nature. Such researches nevertheless merit, 

 as closer study of them teaches, careful con- 

 sideration, and we must not dismiss them 

 without attention. 



For quantitative estimations of tannin we 

 employ material dried at 100 C. Bark, 

 woods, or massive rhizomes are ground up 

 FIG. 114. Extraction very finely in a suitable mill. 2 Delicate 

 apparatus for tannin de- rhizomes or roots, leaves, seeds and seedlings 



terminations. . 



are soaked in water and then rubbed down 

 as finely as possible on a grater. 3 



