38 



In the 3 plants mentioned and in many plants with secondary 

 growth the activity of the cambium cannot long be wholly held in 

 check since, as will be shown later, room for growth will be made 

 by the collapse of cortical cells. When however the space afforded 

 by the collapse of the cortical cells has been filled cessation of 

 growth must again ensue. How long meristematic cells would live 

 while their division was prevented by their inability to extend them- 

 selves there is at present no means of knowing. 



An interesting point noted by Krabbe 1 is that the cambium 

 cells in the trees with which he experimented never divided under 

 pressure till they had reached the size at which they divided nor- 

 mally. Pfeffer in his researches (not yet published) on the growing 

 root- tips found that cells of the primary meristem divided within 

 casts only at the same size as normal, while at a little distance from 

 the primary meristem cells divided at somewhat less than the 

 usual size. 



Measurements to determine these ralations in the plants here 

 described have not been made, because the displacement and distor- 

 tion of tissues render such measurements difficult. 



2. Effect of gypsum casts on the growth of thin-walled 



parenchyma. 



The first effect of surrounding with gypsum a stem in which 

 primary growth has not ended is the filling, by the growth of the 

 parenchyma cells , of the preexisting intercellular spaces. In this 

 early change a displacement of tissues toward the centre may or 

 may not take place 2 . It w r as always present in the Vicia plants as 

 also in Mel Unit lias and in Cucurbita; in Dahlia, Caltha and Althaea 

 it was not found ; while in Urtica, Archangelica, Myrrhis and Riciniis, 

 it was present in some cases, absent in others. The explanation of 

 this difference in behaviour lies in the condition of pith and cortex 

 at the time the gypsum is applied. If the pith has large intercellular 

 spaces the bundles are very likely to be crowded toward the centre ; 

 for the cells of the pith in the plants under experiment enlarge less 

 rapidly and vigorously than those of the cortex, and hence the cortex 

 not only fills the intercellular spaces in itself but closes up those in 

 the pith by crowding the cells there more closely together. In such 



1 Krabbe 1. c. 



2 Aristolochia sipho, as described in De Bary's Vergleichende Anatomie p. 549, 

 furnishes an example of compression of the pith as the result of normal growth. 



