262 SCIENCE PROGRESS 



refuse admittance to sugars ; indeed analyses made by 

 Kraus of the contents of active sieve-tubes of the vegetable 

 marrow have shown that more than a third consists of carbo- 

 hydrates. Fischer, Briosi, Lecomte and others found starch 

 in the sieve-tubes, and more recently Czapek found sugar present 

 in them in most of the plants he examined. In the course of 

 experiments of my own upon various plants I have repeatedly 

 found sugar in the sieve-tubes, often in considerable quantities, 

 even when apparently absent from the other tissues in their 

 immediate neighbourhood. 



Although the presence of sugars and other carbohydrates in 

 sieve-tubes has been recognised for many years, their trans- 

 location by means of these elements, except in very small 

 quantities, seems not to have been generally considered 

 probable. Yet if it be allowed that proteids travel in the sieve- 

 tubes, what is more likely than that the more abundant sugars 

 also follow the same paths during their well-known rapid 

 translocation ? 



There is no satisfactory experimental proof that sugars do not 

 travel in the sieve-tubes and it is simply the neglect of these 

 elements in the past which has led to a rather one-sided treat- 

 ment of the problem. The difficulties attending investigations 

 upon the changes in the amount of sugar in sieve-tubes at 

 various times are not easily overcome by the methods formerly 

 used ; progress has depended on the advent of a more 

 suitable test for sugar than Fehling's solution. Fortunately 

 this is now to hand. 



Meanwhile some account may be given of the kind of 

 evidence hitherto adduced in support of the theories advanced. 

 This has taken various forms, which may be classed as 

 observations of the structure of plant cells and the carrying 

 out of physiological experiments. These will be considered 

 separately, the present article being mainly concerned with 

 cell structure. Before experiments upon the passage of sub- 

 stances within plants can be carried out successfully, it is 

 essential to become acquainted with the internal structure of the 

 parts under investigation. The study of this may suggest 

 possible paths for assimilates, but the actual function of any 

 organ or tissue can hardly be ascertained with certainty until 

 experiments have been performed. Hence it is that much of 



