196 Macfarlane—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 Amphicarpea, 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 Zussilago, 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 Rodinia or Cassia, or this may be: 
