64 SCIENCE PROGRESS. 



existing in special cells which are sharply marked off from 

 the others round them when stained as above described. 

 These special cells vary a good deal in number, apparently 

 according to the amount of the acid present in the plant, 

 and have no very specially regular distribution. Indeed it 

 seems probable that any cell of the tissue may become a 

 centre of deposition of the acid. Generally, if not quite 

 isolated, they only occur two or three together. Certain of 

 the fibres of the pericycle may be observed almost similarly 

 isolated. 



Treub further says that these special cells of the cortex 

 or of the pith derive their supply of hydrocyanic acid from the 

 conducting tissue of the bast and that the amount of them 

 and consequently of the acid varies with the condition of 

 the stem. 



Tracing the hydrocyanic acid upwards through the axis 

 by means of longitudinal sections it can be found to extend 

 throughout its whole length, but to disappear at a little dis- 

 tance from the growing point, the apical meristem of which 

 contains none. 



It is impossible to avoid being struck with the similarity 

 here exhibited to the fate of sugar, amides, etc., which as 

 we have seen can be traced up to the seats of constructive 

 metabolism and there cease, apparently giving rise to 

 protoplasm. If this be so, the hydrocyanic acid must be 

 regarded as a plastic material, unsuitable as at first sight it 

 would appear for that purpose. 



This view is supported by several observations which 

 the author details at some length. He finds that in the 

 apices of young shoots which have suffered an arrest of 

 growth, there are more of the special cells containing the 

 hydrocyanic acid than there are in similar ones which are 

 undergoing rapid elongation. That is, where there is active 

 consumption of plastic material there is no accumulation of 

 the acid, but where plastic substances are compelled to remain 

 unused, hydrocyanic acid is one of such stored bodies. 



Another series of observations considerably strengthens 

 this view, while it points more definitely to the ultimate 

 purpose of the acid. In many of the special cells the latter 



