TRANSLOCATION OF CARBOHYDRATES 475 



theories, their results are really complementary, since the former 

 only followed the sugars over part of their journey, whilst the 

 latter attempted to trace their further path in the sieve-tubes. 

 But as Czapek's evidence is considered by Haberlandt to be 

 insufficient to justify his conclusions it remained to be seen if 

 a more satisfactory proof of the translocation of sugar in sieve- 

 tubes could be obtained. This is not very easy by means 

 of Fehling's solution, its use as a sugar test having several 

 disadvantages. A good deal of diffusion of cell contents occurs 

 in the hot aqueous solution ; delicate tissues are liable to injury 

 on heating owing to the action of caustic potash ; the granules 

 of copper oxide precipitated are very small and not always 

 easy to observe ; crystallisation sets in on the slide and entails 

 a rapid remounting of the tissues tested, making comparative 

 experiments less convenient. 



Moreover Fehling's solution is not reduced by cane sugar 

 and is reduced by other organic compounds which may be 

 present — e.g. tannin. Hence it affords by no means an ideal 

 method of examining the distribution of sugar in plant tissues. 

 A very much better method was introduced by Senft in 1904, 

 by means of which osazones of the sugars can be formed with 

 comparatively little diffusion. These are easily visible and 

 permanent in glycerine. Using this method I have been able 

 to obtain photomicrographs showing the distribution of sugars 

 in the tissues after exposing plants to definite conditions of 

 experiment. Some account of the work was given at the 

 Sheffield meeting of the British Association last year, but the 

 research is still in progress and details must await future 

 publication. 



The depletion of leaves in darkness was studied by examin- 

 ing the sugar distribution in long longitudinal sections of the 

 midribs, larger veins and petioles of various leaves. When 

 the right stage in the emptying was hit upon it was found 

 that while the distal ends of the veins were quite free from 

 osazones, the sieve-tubes nearer the base contained them. 

 The amount present in the sieve-tubes increased towards the 

 base of the veins (cf. fig. 19), and osazones were found freqjuently 

 in the nerve parenchyma. 



Where the vascular bundles of a lateral vein entered the 

 tissue of the midrib the sieve-tubes of these smaller veins were 

 often very conspicuous on account of the bright yellow osazones 



