462 SCIENCE PROGRESS 



very few millimetres of the upper limb and elsewhere no 

 callus had been produced. 



The organic materials used up in the formation of the callus 

 must have come from the tissues above the wound. Czapek 

 therefore held that the distribution of the callus showed that 

 conduction of organic material through the connecting bridge 

 was quite possible in the longitudinal direction, but that such 

 conduction could go on only to a very small extent horizontally. 

 He concluded that the paths of translocation were rectilinear 

 and longitudinal, and that the cells of greatest importance in 

 this process must be those which are longitudinally extended 

 and are connected in strands, i.e. the sieve-tubes and possibly 

 also the cambiform cells — longitudinally elongated cells of the 

 phloem parenchyma. Such horizontal conduction as was found 

 to occur could, he thought, easily be accounted for by the 

 occasional lateral connections of sieve-tubes with each other, 

 and by the very oblique end walls of the cambiform cells 

 which thus touch each other laterally to some extent. 



Czapek adds that although he has so far been unable to 

 obtain direct proof, he " imagines on many grounds " that the 

 sieve-tubes play the most important part in the process of 

 translocation. However, this experiment did not differentiate 

 between carbohydrates and proteids, both of which are needed 

 in the formation of new tissues, and so Czapek next endeav- 

 oured to determine the paths of the carbohydrates alone. 



At the end of bright afternoons he made incisions half-way 

 through the petioles of the leaves of various plants in such a 

 way as to remove half a transverse section of the tissues 

 (fig. 1 8). The leaves were suitably supported and darkened till 

 next morning, when their starch content was examined by means 

 of a solution of iodine. 



The general results were as follows. In plants in which the 

 vascular bundles run separately through the petiole and no 

 cross-connecting strands occur between them, the half of the 

 lamina corresponding to the cut side of the petiole was found 

 to have been more or less prevented from losing its starch. 

 In extreme cases it still contained a great deal of starch when 

 the other half of the leaf had become quite starch-free. 



But where the vascular bundles anastomose, as in ferns, 

 e.g. the lady fern {Athyrimn filix fcmina), or there are cross- 

 connecting sieve-tubes, e.g. the gourd (Cucurbita), it was found 



