108 THE CAUSES OF FLUCTUATIONS IN TUECESCE-NCE
ill very d r y liot weaflier, even if they be subsequently Icept in the slmdc, or in those
detached in cooler and damper weatlier wlieu exposed to strong diroct sunshine after
separation. Eut if t r a n s p i r a t o r y loss be slight, t]ic tissues will retain a considerable
amount of water, especially where atmospheric and t e l l u r i c h i n n i d i t y have antecedently
been high for some time, and this, of course, is available to satisfy the osmotic capaeifies
of the active tissue elements to a greater or less extent. Mere separation from
t h e axis has in no way affected the functional properties of the active tissue elements
remote f r om the site of section, it has only removed them from all connection with
root-supply of w a t e r ; and, so long as the inherent water-conduetiug system retains a
sufticient quantity of water, this is of no importance. Whilst the water-conducting
s y s t em contains a sufficient store of inherent water, it plays tho same part to the
a c t i v e tissues t h a t a moist soil does to a plant as a whole. Tho tissues in the t e r t i a ry
pulvini which make for the diurnal position are f u n c t i o n a l l y much stronger than their
opponents, and hence under continued stimulation by light will undergo a greater increase
i n osmotic capacity than the latter, and as they also present greater structural facilities
for absorption, will appropriate a correspondingly large share of the store of water
r e m a i n i n g at disposal. With this t h e y will increase in strength, and a reversion of the
pinnules towards ihe diurnal position must follow. But if, when this reversion has
been f a i r l y established, the leaf be suddenly exposed to direct sunshine, a sudden rise
i n t r a n s p i r a t o i y loss of water must at once tnsiie, and with this a renewed drain upon
t h e tissues, leading as before to special weakening of those portions of the pulvini
which make for the diumal position. If the activity of titmspiratory loss does not
exceed a certain limit, and if the amount of reserve fluid in the water-conducting
s y s t em be high, this will merely lead to slow, progressive alterations in position of the
pinnules, like that -n-hich occurs in leaves still attached to the axis when exposed
to strong sunshine. But if t r a n s p i r a t o r y loss be very rapid, and the reserve of water
within the water-conducting tissues have been more or less exhausted in supplying
t h e osmotic demands of the actively turgescent tissues, a sufSciently rapid cli'auglit may
bo established to give rise to tho occurrence of rapid resumption of the nocturnal
position. Subsequently, under continued solar stimulation, a rene-wed struggle for any
r e m a i n i n g reserve-water sets in, and a tendency to recovery manifests itseif in greater
or less degree, never, however, leading to any tiling beyond very partial resuuiption of
t h e diurnal position because of the limited amount of water which is aA'ailable, Finally,
as transpiration continues, the conditions for active turgescence are g r a d u a l l y diminished,
and ultimately abolished, and with tliis t h e vaaious parts of the leaf p e r m a r e n t l y assume
t h e positions which are determined by theia- passive structural peculiarities.
During continuous periods of fine, settled weather the i)Iienomena attending separation
of leaves f r om the axis show regular and conspicuous differences, according to the time
of day at which the experiment is conducted. Tho effects of primary separation from
t h e axis and of subsequent exijosm-e to diffuse a n d direct sunshine arc not alike in the
e a r l y morning and in t h e afternoon. When leaves are separated in t h e m<.rning, ere the
dew has evaporated, and whilst the air is relatively humid, tho extent and i-ate of
p r i m a r y assumption of the nocturnal position by the pinnules is less than it is in
t h e afternoon, the amount of primary recoveiy is greater, the tendency to the occurrence
of r a p i d movement on exposure to direct sunshine is less; secondary partial recoverv
f r om the uoctui-nal position after rapid movements constantly occurs in gi-oater or less
r
I IN THE MOTOR ORGANS OP LEAVES. 109 .
degi'GC, and the secondary rachises throughout exhibit little or no movement, in tho
a f t e r n o o n the pinnules assume tho noctural position much more r a p i d l y a n d completely,
p r i m a r y recoveiy is much more limited, rapid resumption of the nocturnal position on
exposure to dii-ect sunshine occurs more f r e q u e n t l y and much more completely; secondary
r e c o v e i y does not follow continued exposure, and tho secondary racliises exliibit more
r a p i d and much more considerable movements of convergence.
Ill order to explain these experimental results on the oixliiiarily accepted theory of the
causation of the movements in Mmosa pudica, it must be assumed either that separation
f r om the axis and exposure to the sun's rays aro stronger stiumli to protopUusmic
contracti<m in the afternoon than in the morning, or that the protoplasts are more
i r r i t a b l e in the afternoon than in the morning; that separation fioin the axis render
t h e cor.traclile protoplasts more irritable or more contractile than they are whilst
retaining thoir normal relation to i t ; that exposure to direct sunshine is, in tho morni
n g , only a temporaiy stimulant to contraction but in the afternoon a permanent one;
and, finally, that the protoplasts of the t e r t i a r y pulvini are much more irritable than
those of the secondrry ones. But any such assumptions have absolutely no foundaiion
beyond the belief, t h a t the movements must be dependent on active contraction of
t h e protoplasts of the motor organs, and some of them are ciii-ectly controverted by
facts. Separation f r om the axis and exposure to tlie sun's rays produce ¿ess effect in
t h e afternoon t h a n in the morning if the air and soil, in place of having become diyer
in the interval, have become more humid; and leaves or shoots separated f r om tho axi
when the soil and aii- aro alike excessively humid are much kss " i i r i t a b l e " than
loaves or shoots attached to the axis M-hen soil and air are both very clay.
T h e other theory can, at all events, point to the existence of differences in
tho
tho
s t a t e o£ the tissues and of the environment corresponding with the differences in
phenomena of movement at different periods. So far as the tissues are concerned it is
evident that in the morning t h e y must normally contain a greater bulk of fluid than
t h e y do in the afternonn. They have not had timo to lose much by transpiratory
loss owing to the relatively high humidity actually prevailing at the time, and they
have previously been exposed to a prolonged period of abolished transpii'ation. Under
these circumstances they are likely to contain a considerable exccss of water within
t h e water-conducting system beyond tho bulk which is absolutely necessary to maintain
active tui-gescence under the existing conditions of immediate transpiratory loss. 'I'hc;
amount of surplus "nill, of com-se, vary very considerably under different circumstances,
but where the humidity of the soil is relatively high, it may well attain such •!
level as to suffice, in coincidence with slight transpii-atory loss, to maintain turgescence
i u the active tissues for some timo aft
supply by separation fi-om the axis, and
-water attending the latter. Under such
demand for root sujiply, as the local ^
voir for a greater bulk of water than is
t o r y expenditure. It is not then at all
t e n t conditions of soil
separation from the axis
q:>]ete arrest is put on any further rootven
iu spite of actual active discharge of
circumstances there is no immediate local
water-conducting system has become a reserimmediately
nd air imply excessi
a t t e n d e d cither by
rcquiied to make good transpira-
•pri.<ing to find that, where previous and exissupply
. entii
ind
absence of movenn
of water,
v e i y feoblG and limited movement only. Where the water-content is such'^'t'haT t hi
to a considerable fall in the liquid
iystem cHrectly, or p r i m a r i l y throughout
active cscape of liquid is suflicient to gitension
present throughout the water-conductiri;