578 LECTURE XXI. 



The periodic variations in the external conditions which 

 are due to the regular alternation of day and night induce, as 

 we have seen, a daily periodicity in the growth of plant- 

 organs, which is conspicuously manifested by those growing 

 foliar organs which perform movements of opening and 

 closing. The daily period of growth closely corresponds with 

 that of tissue-tensions and with that of the variation in bulk of 

 plant-organs (see p. 408) ; there can, in fact, be no doubt that 

 they are all due to the same cause, to a periodic variation in 

 the turgidity of the cells. From our point of view this is the 

 expression of periodic changes in the molecular state of the 

 protoplasm of the cells, of such a nature that its permeability 

 increases during the hours of the day, and decreases during 

 the hours of the night : and so deeply does this periodicity 

 become impressed upon the protoplasm, that, as we already 

 know, the periodic changes continue for some considerable 

 time after the plant has been placed in continuous darkness. 

 The daily periodic movements of growing floral or foliage 

 leaves are brought about in precisely the same manner as are 

 those of leaves provided with pulvini ; the regular recurrence 

 in the evening of the stimulus of the diminution in the inten- 

 sity of light which causes them to close, and in the morning, 

 of the increase in the intensity of light which causes them to 

 open, so impresses itself upon the plant, that the opening and 

 closing movements of its leaves will go on for some time 

 under perfectly constant external conditions. 



The spontaneous variations in the rate of growth, like the 

 induced variations, we attribute in the first instance to varia- 

 tions in the turgidity of the growing cells due to corresponding 

 molecular changes in the protoplasm. 



Leaving the variations in the rate of growth, we pass to 

 the consideration of changes in the direction of growth, and 

 we begin with the induced changes. In entering upon this 

 subject we must bear in mind that, as has already been 

 pointed out (pp. 432, 473, 481), the change is not due to a 

 difference in the effect of the stimulus on the two sides of the 

 organ; the organ, as a whole, is stimulated, and the change 

 in the direction of its growth is its response to the stimulus. 



