196 Macfarlaiic — Current Problems 



propagation of stimulus, varying summations of stimuli, and 

 recovery periods, that are constant for each under the same 

 environmental conditions. These specific variations must be 

 referred ultimately to specific cytological differences in reac- 

 tion of the living substance. 



Again, in the same group of plants, the cells with " aggre- 

 gation " contents that make up the bulk of the irrito-con- 

 tractile centers or pulvini are still very imperfectly understood, 

 but the comparatively sluggish manner in which each aggre- 

 gation mass collects in Drosera tentacle or the pulvinus of the 

 Tick Trefoil, as compared with the lightning-like rapidity shown 

 in cells of Mimosa pulvinus indicate specific cytological differ- 

 ences that are as interesting as they are profound. The 

 nature and function of the crystal cells that surround the 

 bundles, and of the special cells of Haberlandt require addi- 

 tional elucidation. 



Though it is from the botanical side that our knowledge of 

 the tropisms first originated, we have still to determine accu- 

 rately the cytological changes that accompany or originate 

 these. The experimental fact recorded by Dr. Schively that 

 lateral shoots of AmpJiicarpcsa, which are strongly geotropic, 

 may become apogeotropic through no direct action upon 

 them, but from removal of the neighboring main axis above 

 the point of insertion of the branch, points to some funda- 

 mental alteration of the cell substance, possibly in relation to 

 nutrition. The reversal of geotropic response in the peduncle 

 of Tiissilago, and of the heliotropic response in the peduncle of 

 Linaria Cymbalaria are similar growth-reactions whose cyto- 

 logical analysis will be of the highest interest. The conclu- 

 sive demonstration by Pfeffer and Correns of the localization 

 of geotropic irritability in the embryonic cells of the root tip 

 may either indicate a weakening or loss of functional activity 

 in the cells as they become older — resembling the decreasing 

 irritability in older leaflets of Robinia or Cassia, or this may be 



