556 COMPARATIVE ELECTRO-PHYSIOLOGY 



rhythmic excitation. When the tonic condition of the plant 

 falls below par, growth is arrested, however large the amount 

 of formative material present. But if stimulus be applied, 

 while the plant is in this state of growth-standstill, there is a 

 renewal of responsive growth. 



The importance of the absorption of stimulus of light to 

 the response of growth is seen in the development of certain 

 seedlings. These, when grown in the dark, become diseased 

 and perish. The growth and the development of organs cease, 

 even though the cotyledons still contain considerable 

 quantities of unused formative substance. They become 

 moribund, as the expression of their loss of tonic condition, 

 which, as we have seen, depends upon the supply of stimulus 

 from outside. And in the present case the critical element 

 is the stimulus of light. 



This fact that the stimulus of light may enhance the ex- 

 citability of a tissue, is shown in the following photographic 

 records of the mechanical response of vegetal nerve. The 

 first three responses show the extent of normal response to 

 a given electrical stimulus applied in the dark. The specimen 

 was then subjected to the light of an electrical arc lamp, and 

 the next series show the consequent enhancement of response, 

 under the same stimulus as before. Light was now cut off, 

 and after an interval response once more taken. This is seen 

 to have been of the same amplitude as at the beginning 

 (fig. 334). Thus light is seen to enhance excitability. 



In the cases which we have just been considering we have, 

 for the sake of simplicity, confined our attention to a single 

 factor namely the photo-tonic among the many which 

 finally determine the general tonic condition of the plant. 

 And we have found that when the organism is deprived of 

 this source of stimulus, its motile activity, its suctional 

 activity, and its response of growth, all disappear. When 

 the plant, however, is restored to the direct action of light, all 

 these various activities reappear. Now it is clear that this 

 illumination cannot directly penetrate to many of those 

 interior tissues, whose activity is nevertheless essential to the 



