RESPONSE TO STIMULUS OF LIGHT 



407 



consequence of repeated responses there is also a relatively 

 greater contraction and loss of turgidity in that lower r;alf. 

 The leaflets are thus closed by the additive effect of multiple 

 normal negative responses or downward movements. 



When a Biophytum plant is kept in the dark the leaflets 

 undergo a closure which is outwardly similar to that induced 

 by light, but actually arises from a cause precisely opposite. 

 Under strong stimulus of light a general contractile effect 

 drives the water inwards to the interior of the plant, and, the 

 loss of turgor in the lower halves of the pulvini being, as we 

 have just seen, greater than in the 

 upper brings about the closure 

 of the leaflets. But in complete 

 darkness the reverse process is set 

 up. The water returns to the 

 pulvini, making them over turgid. 

 Under this condition of excessive 

 turgor, however, the upper half of 

 the pulvinus becomes more turgid 

 than the lower, and, being thus 

 rendered more convex, closure of 

 the leaflets is brought about Ex- 

 posure to light in this condition 

 induces a greater loss of turgor by 

 the upper than by the lower half, 

 and we consequently obtain the 

 outspread or erected position of the leaflets by a series ot 

 small positive movements. Thus, as the result of internal 

 changes, the response of an organ may undergo reversal 

 from the ordinary negative to positive. The change from 

 light to darkness, by which we have here seen the character 

 of response to be so greatly modified, occurs in the diurnal 

 periodic alternation. And we can see that, in consequence 

 of such an imparted periodicity, an alternation of phase is 

 impressed upon the organism, which will carry with it a 

 periodic variation of excitability. If this diurnal periodicity 

 acted alone its after-effect would be comparatively simple, 



FIG. 241. (Multiple Mechanical 

 Response of Leaflet of 

 Biophytum under the Con- 

 tinuous Action of Light 



