460 SCIENCE PROGRESS 



between the development of the tissues above and below 

 the ring. Such a result indicates that the phloem plays an 

 important part in the conduction of the substances required 

 to form new tissues. 



Lecomte paid particular attention to the effects of ringing 

 twigs upon the accumulation of starch in the various tissues. 

 Above the ringing it became very abundant in the cortex, 

 the phloem parenchyma, the medullary rays and the external 

 region of the pith. The corresponding tissues below the 

 ringing proved to contain very much less starch or to be 

 nearly starch-free {Vitis viiiifera, Cerasiis PaduSy Quercns Robiir). 

 At the level of the interruption the medullary rays of the wood, 

 and also the pith, showed very little starch. Furthermore 

 in the vine, the sieve-tubes of which ordinarily contain starch, 

 no accumulation of this could be observed, though large 

 quantities of albuminous substances were demonstrable. 



These experimental results led Lecomte to conclude that 

 the sieve-tubes do not appear to conduct anything but albumi- 

 nous substances, that carbohydrates are contained in the 

 parenchymatous tissues, and that they travel from one cell to 

 another in the various types of parenchyma occurring in the 

 stem. Moreover, the very small amount of starch in the pith 

 and the medullary rays of the wood at the level of the wound 

 seems to show that in these tissues conduction only goes on 

 radially. 



Lecomte thought that if carbohydrates travel in the sieve- 

 tubes they would accumulate and form starch under the above 

 conditions. But Czapek, who repeated these experiments in a 

 modified form and obtained the same general results, pointed 

 out that the storage cells with which the sieve-tubes are 

 connected do accumulate large quantities both of starch and 

 of nitrogenous assimilates. He considered that the failure of 

 starch to accumulate in the sieve-tubes is no argument against 

 the theory that these elements serve to conduct soluble 

 carbohydrates, since starch is only formed when the con- 

 centration of the sugar exceeds a certain value. The surplus 

 sugar is then deposited as starch, which thus represents a 

 reserve, rather than carbohydrate in course of translocation. 



As shown by the researches of Meyer and of Schimper the 

 point of formation of starch from sugar varies greatly in the 

 leaves of different plants ; accordingly they can be divided 



