474 SCIENCE PROGRESS 



From these and other considerations the conclusion was 

 drawn that ** one function of latex is to carry reserve materials 

 in the plant." 



Although the method used is not altogether free from objec- 

 tions (cf. Parkin, 1900), one that is has yet to be discovered 

 and in its absence no very definite statements can safely be 

 made with regard to the extent to which conduction takes 

 place in laticiferous tubes. 



It has already been stated that Czapek showed the existence 

 of sugars in the sieve-tubes of numerous plants, but in addition 

 he made an experiment with the object of proving by micro- 

 chemical means that these sugars travel in the phloem. A 

 gourd plant was darkened during nearly a week to allow the 

 leaves to empty of their carbohydrates. Then one leaf was 

 exposed to bright sunlight during eight hours, the rest of the 

 plant being kept in darkness. At the end of this time the 

 exposed leaf was cut off and the iodine test showed that starch 

 had been formed in it. Transverse sections were then cut from 

 the petiole of the illuminated leaf and from those of the 

 darkened ones, and these were tested for sugar by means of 

 Fehling's solution. The result was that the part of the bundles 

 containing sieve-tubes in the petiole of the leaf which had been 

 assimilating became markedly distinguished from the other 

 tissues by the presence of a considerable quantity of red copper 

 oxide, formed presumably by the action of reducing sugars. 

 No such distinction between the tissues could be made out 

 in the case of the petioles of the darkened leaves. The results 

 were therefore taken to indicate that the sugars are translocated 

 in the phloem, though Czapek did not in this place explicitly 

 state that the sieve-tubes themselves are the conducting 

 elements. 



The difficulty in an experiment of this nature is to secure 

 proper controls; it is indeed practically impossible, as the same 

 tissue cannot be examined both at the beginning and at the end 

 of the experiment. Individual leaves empty at different rates : 

 even after ten days in darkness I have found sugar present 

 in the sieve-tubes at the base of one of the larger veins of 

 a Cucurbita leaf. But the results of Czapek's experiment 

 certainly do in a measure justify his conclusion. 



Although Schimper and Czapek were led to support different 



