Silberberg: Stimulation of storage tissues 495 



control, but irregularly and in large masses (see figure 3). These 

 pieces were also more susceptible to attacks of bacteria and moulds 

 than were the control. In the control pieces callus was formed 

 more slowly, but in a uniform layer over the entire surface of the 



specimen (see figure 2). In two cases, the control had more 

 callus formed than the poisoned pieces. In the other four experi- 

 ments the results varied on the different occasions when the speci- 

 mens were examined, making them valueless. Solutions stronger 

 than tenth or twelfth normal inhibited the formation of callus. 

 Those weaker than fourteenth normal, seemed to have little, if any, 

 effect upon the callus formation. 



Since the results in the formation of meristem in the potato 



r 



were, by far, the most constant and satisfactory, the experiments 

 on the potato will be described in detail : 



Experiment L Normal and half-normal solutions of zinc sul- 

 phate w^ere used. 



After four days ; 



Control. — One row of meristeniatic cells formed beneath the 



dried exterior. 



N. — No meristem. 

 N/2. — No meristem. 



After seven days : 



Control. — A well-defined layer of meristem. 



N. — No meristem, 



N/2. — A few scattered meristematic cells two or three rows 

 of cells beneath the surface, which was deeper than the location 

 of the meristem in the control. 



After fifteen days : 



Control. — A meristematic layer several cells in thickness. 

 N. — Looked as if it may have recovered from the poison 

 about two days before, as there were a great many scattered 



meristematic cells. 



N/2. — A distinct layer of meristem a Httle beneath the 



surface. 



Experiment IL Fourth- and eighth-normal solutions of zinc 



sulphate were used in this experiment. 

 After four days : 

 Control. — A distinct meristematic layer forming. 



