AUTONOMIC MOVEMENTS OF OPUNTIA. 



39 



be formed which raised only the extreme terminal portion of the stem and 

 which slowly moved down the stem. Meanwhile, the basal portion of the 

 stem was still in the condition of having a concavity on the opposite lower 

 side, so that when the curvature now under description had progressed to the 

 middle of the stem in its downward course, the shoot or branch was in the 

 form of a lax S, with antagonistic curvatures in the apical and basal por- 

 tions. It was in this position that the marked minor movements occurred. 



Eventually the course of the progressive curvature was continued, and 

 at the end of two days it was sharpest at the base. Now, however, the 

 curvature which would carry the branch toward the window, instead of be- 

 ginning at the tip, would take place on the opposite upper side of the basal 

 portion of the stem and gradually move upward, with the result that by the 

 time it had reached the middle of the stem the tip was brought into the posi- 

 tion directed toward the window. The movement away from the window 

 was by a curvature beginning- at the tip and extending downward, 2 days 

 being necessary for it to reach the base of the organ and with antagonisms 

 resulting in minor movements in the median position. The movement 

 toward the window was produced by curvatvires beginning at the base of 

 the stem immediately opposed to the curvature, which had just reached 

 that region on the opposite side. 



Nutations are not easily interpreted, as evidenced by the many laborious 

 attempts made in explanation of them. Positive information does not go 

 beyond the fact that tropistic reactions, particularly to gravity, and the 

 irreversible changes in form due to rhythmic growth-action are involved. 



Alternation of position of the zone of rapid growth from one side or flank 

 to another is well known, but alternation by which this zone moves up one 

 flank and down the other is quite anomalous. Ink-marks were placed 

 near the base, at the middle, and near the tip of the shoot on the side 

 away from the window, and the two intervals thus laid off were measured 

 from time to time in October. A caliper-scale with vernier dividing the 

 millimeter to tenths was used. The movement now made a cycle which 

 was completed in about 2 days, so that one set of measurements would 

 be in the concavity and the distance between the same points on the fol- 

 lowing day would be obtained by the application of the ruler to a surface 

 now convex. 



