RESPONSIVE ACTION OF PLANT-TISSUES TO LIGHT 573 



undergo rapid diminution till arrested, as seen by the curve 

 becoming horizontal at its highest point. The continued 

 application of light now proceeds to cause a marked contrac- 

 tion, the maximum rate of which is -02 mm. per minute. It 

 is therefore to be noticed that, in this particular experiment, 

 light not only retarded growth, but also produced an actual 

 shortening of the plant. On the cessation of light, growth was 

 slowly recommenced, but the average rate of growth during 

 the sixteen minutes following the stoppage of light was only 

 •001 mm, instead of the normal rate of 015 mm. per minute. 

 This rate was, however, gradually increased in the absence 

 of light, and after a certain interval became normal again. 



It must be borne in mind, in connection with this, that 

 though the immediate and after-effects of incident light are 

 here seen as retardation of growth, nevertheless the rate, 

 after the lapse of a certain interval from the cessation of 

 stimulus, may be again increased above the normal in con- 

 sequence of the enhancement of the tonic condition of the 

 plant, by its absorption of energy of light. 



In the last experiment we saw that responsive contraction 

 not only arrested growth, but made the tissue actually 

 shorter. In other instances the contraction and resulting 

 retardation are not so great. Thus in a second experiment 

 with Sinapis, the normal rate of growth was -02 mm. per 

 minute, which during the continuance of light fell to one- 

 tenth of this, or -002 mm. per minute." Thus the effect of light 

 on the growing organ is always a contraction, which may 

 in some cases induce a mere retardation, but in others 

 culminates not even in cessation of growth, but in an actual 

 shortening of the responding tissue. 



The effects thus described occur in plants which arc in 

 normal tonic condition. But we have seen that when the 

 specimen is, on the other hand, in a sub-tonic condition, 

 absorption of enegy in any form from outside will, at first, 

 by increasing the internal energy, serve to accelerate growth ; 

 and that afterwards, when the normal tonic condition has 

 been attained, external stimulus will have the normal effect 



