45 6 



PHYSIOLOGY 



of position, which are executed by differences of turgor on opposite 

 sides of motor organs appropriately situated. Inasmuch as the changes 

 in illumination are not sudden (in nature), it should be expected that 

 the movements would not be restricted to morning and nightfall. In 

 fact it can be shown that there is really a slow variation, so that in the 

 brightest hours of the day the blades reach their highest or lowest posi- 

 tion, the opposite being attained in the maximum darkness. As the 



changes in the inten- 

 sity of the light are 

 most marked at dawn 

 and at dusk, the 

 changes of position 

 are then most rapid 

 and so attract atten- 

 tion. 



Persistence. — To 

 these periodic vari- 

 ations in light the 

 plant becomes habit- 

 uated, and even if 

 they are not allowed 

 to occur, as when a 

 plant is kept in con- 

 tinuous darkness or 



Figs. 686, 687. — Shoot of the purslane {Portulaca olcra- . ,. , , 



cea), photographed from identical position at 2 p.m. (686) continuous llgnt, tne 

 and at 8.30 p.m. (687); note that the older leaves show little movements continue, 

 change of position. - From photograph by Land. ^ diminishing am _ 



plitude, for a considerable time (3-5 days) before they cease entirely. 

 The normal periodic stimulation seems to have impressed upon the 

 protoplast a rhythmic variation in turgor, so that it cannot at once 

 cease the customary action even when no stimulus demands a reaction 

 (fig. 688). 



When these movements are ceasing, there come to view similar ones which are 

 usually masked by the photeolic reactions. These, however, are autonomous ; 

 they are much less extensive and have a much shorter period than the others. 

 When sought, they can be observed even in the presence of the photeolic movements. 

 They consist of a pendulum-like swinging of the leaf or leaflets, up and down (some- 

 what as in Desmodium, fig. 684; see also fig. 689), whose advantage and effects 

 are alike obscure. 



