METABOLIC PROCESSES IN THE PLANT. 335 



For extraction of the tannin the arrangement represented in 

 Fig. 114 is very suitable. The apparatus, which is made of tin, 

 consists of a lipped cylinder 12'5cm. in height and 7 cm. in internal 

 diameter, and of a rod carrying at one end a sieve plate. Before 

 using the apparatus, a piece of thin gauze is very carefully tied 

 over the sieve. 2-5 gr. of the research material, dried at 100 C., 

 are treated with 200 c.c. of water, and after twelve hours the fluid 

 is poured into a litre flask. The residue is now transferred to the 

 extraction apparatus, and four times digested at boiling point for 

 half an hour each time, with 200 c.c. of water, the cylinder being 

 placed for the purpose in a water-bath. The extract is finally 

 filtered. The tannin in the filtrate is accurately determined by 

 the Lowenthal-v. Schroder method, as described in the Report 

 cited in Note 3, by titrating with chameleon solution [standard 

 solution of Potassium permanganate] and Sodium sulphindigotate 

 (Carminum ccerul. opt. of Gehe and Co., Dresden). The volume 

 of the tannin-containing solution used in any particular titratioii 

 must be such that the volume of the chameleon solution needed 

 for its reduction is not much less than that required for the indigo 

 solution (10 c.c.) alone. From the volume of chameleon solution 

 required is determined the percentage of tannin so-called, 011 the 

 assumption that 1 c.c: of the chameleon solution corresponds with 

 2 mgr. of tannin. 



Among the most important facts determined by Kraus in his 

 cited work are these, that tannins are produced in green leaves in 

 the light and in presence of Carbon dioxide, but not in the dark 

 or in absence of Carbon dioxide. Nevertheless, tannins are not 

 to be regarded as products of assimilation ; more probably they 

 originate as bye products of metabolism in the synthesis of 

 proteids. Many plants, moreover, in general do not form tannins, 

 and often even in organs which normally contain them they 

 are not produced, although carbohydrates are being formed by 

 assimilation. This last is the case, e.y., in comparatively feeble 

 light. 



We cut in summer, in warm weather, six mature leaves of 

 Saxifraga crassifolia, remove one half of each leaf by section 

 alongside the midrib (without, however, injuring this), and from 

 the isolated halves remove pieces, each about 50 sq. cm. in area, 

 using templets made of millimetre paper. We now dry these 

 pieces of leaf, and determine the quantity of tannin they contain. 

 The half -leaves left attached to their midribs are placed with 



