THE CAUSES OF FLOCTUATIOKS IN TURGESCENCE
lurgGscenco is o g m d m l om, and whom root-anpply is abundant and almosplieiic hnniidi
t y IS not excoasively dopressod, a modified diurnal position is shortly arrived at, the
precise nature of wliioh varies ,vith t h e relations between supply and loss oi water, the
dilîcreneos i n osmotic properties of the opposed masses of p u l r i n a r tissues, and any alterations
m the l e r a - a g e of distal parts which may have been induced. The reserve-water,
or the water continuously supphod fcom the roots, acts as a kind of buffer inlerjiosed
between teansph-atory loss and the water of tnrgesconce contained within the active
tissues. The increased transpiratory loss must, of course, eventually tell on the amount
of water available for tho up-koep of tm-gesceiice in all cases where root-supply is absent,
or IS incapable of increasing in a c t i v i t y to tho extent of e n t i r e l y discounting tho increased
loss; but m the presence of a large stock of reserve-water or of active root-supply it
only does so gradually, whereas in the oases of detached leaves, unprovided with much
reservc-wulcr, an active drain sets in at once upon the turgescent tissues, and specially
on those which provide the greatest sti-uotmal facilities for the redistribution of liquids.
But in certain cases raiiid sudden movements do a t t e n d the sudden exposure of
attached leaves to dii-cct sunshine. In hot dry weather, it not u n f r e q n e n t l y happens that
when tho dii-cct sunshine first strikes upon a plant in the inoi-ning a certain number of
t h e youngest mobile leaves, in which full e.xpansion of the pinnules has not yet been
established, presently show rapid movements of depression in theii- p r i m a r y petioles. The
movements hi such cases may bo owmg to the action of the same cause as t h a t which
occasions sudden movements in detached leaves. They may be owing simply to the
sudden increase in transpiratory drain, acting upon the pulvinar tissues which make
f.ir the diurnal position at a time when they are specially structurally weak owmg to
t h e i r incomplete development; but it appears probable that, in port Bt all events, they
owe their origin to the action of another factor. Tho delicate tissues of the terminal
portions of the axis on which the youngest leaves are situated arc, of course, v e r y readily
a i f e c t e d throughout by sudden changes in temperature, and just as a sudden and considerable
fall in temperature must tend to establish a back-draught of liquid into the
water-conducting system, so a sudden and considerable elevation may lead to tomp
o i a i y obstmction to its onward progress. Any sudden elevation oi temperature must
occasion con-esponding expansion in all the gaseous contents of tho water-conducting
system, andi,if the expansion exceed eertam limits,' it must almost inevitably lead to
more or less obstruction of the carrent of water ascending from the roots: It is not,
however, necessary to remain contented with theoretical considerations, for there il
conclusive experimental evidence which demonstrates not only the occmu-ence of such
obstruction, but also the fact that it is amply sufficient to give rise to the occarreneo oi
sudden and extensively propagated movements. If an open «ame, or the points of a
heated paii- of forcep.s, be ajiplied to a n y point in the course of an axis of Mimosa
jiudica, or, still better, if the sun's rays be accurately focussed upon it, very conspicuous
phenomena of movement presently manifest them.seh-es in the leaves, unless under conditions
of excessive saturation of the soil and atmosphere. In the case of young, soft
shoots t h e treatment gives rise to conspicuous local change in coloar and to more or less
conspicuous discharge of liquid upon the surface, but in mature, hard shoots little change
of colour occurs unless actual charring of the tissues be effected, and there is no exudation
on the surface. These differences are satisfactorily explained by tho différences in
t h e nature of t h e tissues in the two cases. In the young shoots a delicate epidermis
provided with stomatic orifices is present covering a mass of young active hypodcrmal
IN THE .MOTOR ORGANS OE LEAVES. 113
tissue; whilst, in the mature axis, epidermis has been r e p k c c d by strata of corky tissue
p r o t e c t m g the deeper portions of hypoilerina with a dense impermeable coat, and
t h e r e IS no system of great intercellular channels, like that in the iiypoderma of tlie
petioles, to afford accommodation to fluids escaping from the interior of tho tissue
elements (Plate V, Figs, 8, 15). The phenomena are the parallels of those occurring in
ca.ses of exposure of shoots to the action of chloroforni-voponr in which suiierliiini
exudalion of liquid occurs in young, green shoots and is enrircly absent in mature
woody ones. Emining parallel with these differences in local cSecls, wo find differences
i n irapcct to the phenomena of movement following tlie t r e a t m e n t . In tho ca.so of tlio
young, soft shoots movements may occur both ceutripetally and centrifogally along tincourse
of the axis, just as they occur both c e n t r i f u g a l l y and centripetally after the
a p p h c a t i o n of heat to points in the course of petioles; but in old, woody shoot, thev
, occur m centrifugal direction alone, miless the point at which heat is anplied I,e i l
m e d i a t e l y in tho neiglibourhood of the first leaf m the centripetal d i r e c f e n in „1 r h
case movements may occur in it, but in it alone in that direction (vid. appendix F).
S o w it is not easy to account for those phenomena of distribution of movement in
m a t u r e shoots, save as t h e result of local obstruction to water-supply. Where the Z ^ l
a r e still young and soft, heating loads, not merely to s u d d o n ' t e a l e x i ' s i ^ o t ie
gaseous contents of the tissues, but to a sudden loss of water of turgesceuce from t l e
outer surface, precisely sindlar to that a t t e n d i n g immersion in boiling water or death due
o exposm. to the vapour of ehloroform, and in the ease of p'cioles, a l t h l n ! ™v
ii-ee escapo o± Jiquid of turgescence does not occur on tlio outer surface ainnle
modation for such liquid is afforded by the great hypodermal, i.iteroc'iula. . . h a T e k -
but in the hard dense tissues of old axes the suborous strata prevent s„ Z i
exudation, and the density of the texture throughout the hypoderma ailoids no
space for any appreciable balk of liquid save within the i l r i o r ot Z T J
a n d hence any sudden rise in temperature is practically expended in .rivin., rise to
corresponding expansion of the gaseous contents of the water-conchotin.r system
Where any appreciable exudative discharge of liquid of turgescence takes n l a i a^
i n the case of youug soft shoots, two factors afi:ecting water-supply come m ,
p l a y ; there is both sudden expansion of the gaseous contents of tlfe w a t L"
d n c t m g system, and a sudden est.blislmient of an abnormal local drain of '
Any obstruetiye action of the f o m e r can only tell in i n t e r f e r i n g with w a t e Z t
m a centrifugal direction unless in the immediate neighbom-hood of the site in w l l
It originates but the abnormal local drain attending exudation must, of coi ™ ¿e
capable of telling both centrifugally aud centripetally. There are t L s Jl
movemen s dependmg on sudden depressive alterations in water-supply should i n such cases
make theii- appearance m both directions. But where, as in old s loots, no local I n d Z n
occurs, whore no local di^in is established, we have to deal with obs u l n
.oy can only occur contrifugaily or in the i a i . u e d i a t o ' n e i g h b o i i w r t l m r i n " " : ^ ^ ^ ^^
t h e obstruction o r i g i n a t e , because everywhere beyond thi, area in t h e oeu,ripe,.°i fecriou
A.nx. Eot, Bor, Q.hd. C.iLcci-rA Tm.. Tf.