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 
.nub. ooo, 007. ouuui 01 me jjursiaiie \furnuutnt uicru- i. i , .1 
ceo), photographed from identical position at 2 P.M. (686) C U S M > ll 
and at 8.30 P.M. (687); note that the older leaves show little movements continue, 
change of position. - From photograph by LAND. ^ ^^5^ 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. 
FIGS. 686, 687. Shoot of the purslane (Portulaca olera- 
