AUTONOMIC MOVEMENTS OF OPUNTIA. 



41 



gradually to the tip. The irregular variations seen in the major movement 

 are to be expected in tissues undergoing- the changes finally leading to decor- 

 tication. Similar irregularities are seen in the ordinary type of nutations. 



Fig. 1. A, diagram of cross-section of etiolated shoot of Opuntia blakeana, showing 

 number and position of fibro-vascular bundles and volume of parenchymatous 



tissue. B, cross-section of etiolated shoot which has been exposed to light, with 

 shrinkage of the outer "cortical" layer. 



It seems very clear that these autonomic movements are not nutations. 

 They ensue only within a certain range of temperature and are undoubt- 

 edly due directly to the disturbance of the balance of turgidity in mov- 

 able zones on opposite sides of the shoots. The shoots were held rigidly 

 in all positions, which leads to the suggestion that the disturbance of the 

 balance must be attributed to a heightened turgidity on one side, although 

 the possibility is not wholly eliminated that a slight reduction of turgidity 

 on the concave side might be the actual cause, without lessening the 

 rigidity of the stem to a perceptible extent. In fact, nothing but purely 

 speculative reasons might be given to support the conclusion that an in- 

 crease of the osmotic activity of the contents of the cells of one side, or a 

 decrease in the other, was the actuating cause. The method of develop- 

 ment of a rhythm by which the alternation of movement ensued would be 

 the same in both cases. 



It is to be noted that these movements take place when a plant is brought 

 into intimate relation with an environmental factor represented by the 

 host plant. It is probably connected with the restricted water-supply 

 obtained from the host. No aspect of the phenomenon in question suggests 



