40 



which have formed the material for the present work have in no 

 case shown thicker walls within the casts, hut on the contrary have 

 shown in general thinner walls in pith and cortex. An individual 

 of Ricinus communis an internode of whose stem was wound with 

 twine as Wortmann wound his plants showed however after growing 

 for some weeks in this condition a band of collenchyma in the 

 cortex with walls vastly thicker than the same stem showed in nor- 

 mal positions. In this case the peripheral cells of the cortex had as 

 in Wortmann' s plants grown in among the coils of the twine. 



In the plants that had made good growth after the gypsum casts 

 were applied there was always in the vicinity of the upper limit of 

 the cast thicker cell-walls than normal. This abnormal thickening 

 could always be detected in pith and cortex cells, being strongest just 

 above the limit of the cast and decreasing upward and downward 

 from that position, reaching within a centimeter above the limit of 

 the cast the normal condition, and a centimeter down into the cast 

 the normal or a condition in which the cells had thinner walls than 

 normal. 



It is thus seen that in these plants the thickening of cell- 

 membranes occurs within the cast only near the upper limit. Longi- 

 tudinal sections show that the cells in the upper centimeter of the 

 cast have undergone extraordinary gliding growth, while farther down 

 in the cast such has not been the case. Just below the upper limit 

 of the casts the walls of the cortical parenchyma that normally 

 are nearly horizontal are inclined from the periphery toward the 

 centre upward at an angle of 45° or more. Passing downward in 

 the stem the inclination gradually becomes less till at the depth of 

 about a centimeter the horizontal position is found. The outer cells 

 of the stem have been held by the gypsum while the more internal 

 by their efforts to elongate have undergone displacement upward. 

 Thus it seems probable that the tension of tissues has been the sti- 

 mulus for producing thicker walls. In the case of the Ricinus plant 

 cited above and in those plants described by Wortmann it is not 

 improbable that more than the normal tension arose in the cortex 

 either by longitudinal growth of the part held by the band or by 

 the uneven growth of the peripheral cells into the band, and thus 

 stimulated the thickening of cell-membranes. That tension may stimu- 

 late such a formation is well known, especially from Pfeffer's l review 

 of an unpublished research by Hegler. 



1 Pfeffer, R. Hegler's Untersuchungen iiber Einfluss von Zugkraft. Berichte 

 d. k. sachs. Gesellsch. d. Wissensch. December 1891. 



