DUAL EFFECT OF STIMULUS OF LIGHT 55 



nating — that is to say, one response is large and the next 

 small, in regular sequence ; after a time these tend, however, 

 to become uniform. By the employment of the High 

 Magnification Crescograph I find that growth consists of a 

 series of pulses, which are often of an alternating character. 

 The same is true of the autonomous pulsations of the leaflet 

 of Desmodium gyrans. By the employment of a sensitive 

 Bubbler which records successive evolutions of i c.mm. 

 of oxygen, I find that the photosynthetic response of Hydrilla 

 is sometimes of a similar character, a short period alter- 

 nating with a longer interval. These, as in the alternating 

 responses in Mimosa, become uniform after the attainment 

 of a steady condition (fig. n). The parallelism here ob- 

 served in the two different modes of response is indeed very 

 remarkable. 



Dual Effect of Stimulus of Light 



Light induces both the A and D effects, the resultant 

 being A— D. The C0 2 -assimilation by the leaf is essen- 

 tially an anabolic process, and the predominant response 

 of an actively assimilating leaf is A ; in regard to D, it is 

 increased under strong intensity of light, with the resulting 

 diminution in the rate of photosynthesis (p. 36). 



In other instances, the negative catabolic D may be 

 predominant and mask the less predominant positive A. 

 It is, however, possible to unmask A (under circumstances 

 when it is more persistent) by the sudden stoppage of 

 light. 



Unmasking of A in mechanical response. — In excitable 

 growing organs photic stimulus, giving rise to the predomi- 

 nant D reaction, retards the rate of growth ; but I hoped 

 to unmask the A effect on the stoppage of illumination. 

 The investigation was undertaken with the Balanced Cresco- 

 graph, in which the upward growth is exactly compensated 

 by the subsidence of the plant at the same rate. Hence the 

 tip of the growing organ remains at the same height, and the 



