

MOVEMENTS OF IRRITATION. 



181) under the influence of light assume a nearly horizontal 

 position,* whereas in the absence of light they lay themselves 

 together downwards. The leaflets of Phaseolus exhibit also 

 periodic after-effect movements, when plants which have first been 

 growing under normal conditions are then placed in constant dark- 

 ness. For observations on this point, we require vigorous pot 

 plants. In investigations which I conducted, the periodic after- 

 effect movements lasted several days, though certainly with dimin- 

 ishing amplitude. The leaves finally became darkness-rigid, in 



FIG. 180. Leaf of Phaseolus multiflorus, day 

 position. 



PIG. 181. Leaf of Phaseolus 

 multiflorus, night position. 



which condition they were expanded horizontally. Once more 

 exposed to normal conditions, the younger eaves speedily returned 

 to the phototonic condition ; the older ones evidently reacted far 

 less energetically to the stimulus of light. 



In the production of the periodic movements of not fully-grown 

 leaves of Phaseolus, processes of growth certainly play some part. 

 The movements of mature leaves of Phaseolus, however, as is also 

 the case in many other plants (Mimosa, Oxalis, etc.), are entirely 

 due to variations in the amount of turgor-expansion. And these 

 changes causing the movements show themselves in the joints of 

 the leaves (the pulvinus of the main leaf -stalk as also the pulvini 



* This takes place, however, only in bright diffuse light. In direct sunlight 

 the leaflets assume, particularly at noon, a position which in some respects 

 resembles their night position. 



P. P. 



M M 



