458 SCIENCE PROGRESS 



It will be convenient to deal with each type of experiment i 

 separately and to show how it has been applied to the various 

 tissues under consideration. 



Experimental Interruptions of Conducting Channels 



The earlier experiments of this kind took the form of 

 removing a ring of tissue from a woody plant as far down as 

 the cambium and then observing the amount and nature of 

 the subsequent changes in the portions above and below the 

 ringing. 



Hanstein, in i860, employed this method and obtained a 

 number of interesting results. He found that a cut twig of 

 a dicotyledonous tree if placed in water or moist air develops 

 roots at the cut end and a few leaves or shoots above. But if 

 a ring of tissue be removed quite near the base roots only 

 develop above it, whilst if a vertical connecting strip of tissue 

 be left the twig behaves as if no ringing had been made. 



In the case of a shoot which has already produced roots at 

 its cut end a ringing just above them prevents their further 

 development and new roots form above the ringing. When 

 the tissue is removed at some distance from the lower end a 

 few small roots develop below and are the more numerous and 

 well developed the greater the distance of the ringing from 

 the end of the twig. 



If, however, the operation be performed on monocotyledo- 

 nous plants (which have scattered bundles) or on any plant 

 having strands of phloem internal to the zone of wood, as a 

 rule roots develop both above and below the ringing. The 

 growth of those below shows a strict relation to the relative 

 amounts of internal phloem not interfered with by the operation. 



Hanstein therefore concluded that the unthickened tube-like 

 cells of the phloem are the characteristic organs for the con- 

 duction of formative sap, whilst the cambium and the thickened 

 bast cells are at least not essential, and the parenchyma is not 

 sufficient, for such conduction. 



However, his experiments were not considered by Sachs to 

 be altogether conclusive. Sachs contended that if Hanstein's 

 "conducting cells" really conducted all the assimilates they 

 ought to contain starch as well, which was rarely the case. 

 As the point will be dealt with again it will be sufficient to 



