6 4 THE CAUSES OP FLUCTUATIONS I S TUEGESCENCE
Somewliat lalor, and just before nyctitropic morements beghi to appear, the folded piimas
still remain only Tery slightly divergent, bnt in place of being elevated they are now
permanently depressed, the midribs sloping downwards and forwards from the line of
the primai-y petiole and having their koals directed downwards in place of outwards
(Plate III, Fig. 1). The secondary petioles now show distinct evidence of an accumulation
of pulvinar pai-onehyma over the anterior surface basally and along the entire length of
the outer surface. Movements soon begin to appear and to lead to gradually
diurnal elevation, divergence and espansion of the pinnaj, alternating with i
)CtiirnaI tltiiiinie
pression, convergence, and folding up of the laminm. Somewhat later the lar
becoint
permanently expanded, and at the same tin>e alternating movements of d.vergmce and
elevation and of convergence and depression go on iucreasing m amount. At this peiaod
the pinnai during the day are wi.icly divergent, and have their upper surfaces approximately
in the same plane as the upper sniiaca of the rachis (Plate III, Fig. 4), and at
ni.'iit are very closely convergent and depressed so greatly that their upper surfaces lU-e
vertical to ihe lino of the rachis and look directly forwards towards its apex. Tlie nocturnal
position at this stage may bo regarded simply as the result of extreme convergence
and depression wdtheut any approciablo rotation. Rotation of the lamina, however, soon
sets in and, for a time, goes on gradually increasing until, for a period, the upper surfaces
of the pinnso eome to look directly inwards facing one another. This is the penod at
wliich the movements attain their maximum development, and it is succeeded by one in
wliich they undergo gradaal diminution. The cUminution in rotation causes the upper
surfaces of the lamina: nocturaally once more to be directed more
nd more forwards,
reverting as they do so to a position more or less like that proper
> an earlie
of dovclopment. 'I'he reversion, however, is not complete, as it is interfered with by
the coincident diminution in convergence and depression. Finally, a period arrives at
which the progressive diminution loads to a complete abolition of all appreciable movement,
and the pinna: remain permanently fixed in what was previously the dmrnal
position with the upper surfaces of their lamina! looking directly upwards in the plane
of tlie rachis or sightly ascending from it, and their mid.-ibs at right angles to it, or
even somewhat inclined backwards towards the base of the leaf.
On examining the secondary petiole of a pinna at the period at which movements
are at their maximum, two distinct masses of pulvinar tissue may be reeogmsed m it.
They ilifier from one another in tint, the one having an ochreous tinge, whilst the other
is pru'e green, and they are arranged in a peculiar spiral fashion (Plate III, i'lg. 2, 3).
The ocln-eous tissue at the base of the pulviuus lies antero-external, and farther out
it becomes purely external; whilst the green tissue basally is purely axdlai-y, bat towards
the laminar extremity invades the anterior surface of the pulvmus
as to become
oontinuoui with the mass of green parenchyma wdiich forms a lidge over the .
midrib. The pulvinns extends for some distance into the base of the lamina on the
under-surfacc, and the green tissue on the inner side of the midrib is more conspicuous
in amount than the ochreous tissue on rhe other side.
B of the
It is this spiral disposition of the opposed masses of palvinar parenchyma that
determines the peculiar character of the movements of tl.e pinna!. When the development
of the pnlvinus first begms, the fact that the tissue, which is subsequently distinguished
by its ochreous tint, makes its appearance first accounts for the permanent depression
and convergence of the pinna which prevails immediately antecedent to the period at which
IN THE MOTOB ORGANS 01? LEAVES. 65
movements first begin to appear. The depression is determined by the aeoumnlation
of parenchyma over the anterior aspect of the base of the secondary petiole, and convergence
by the extension of it along the course of the outer side. Wlien pulvinar parenchyma
begins to accumulate beneath the base and along the axillary surface of the secondaiy
petiole, and to extend thence over the anterior surface of the base of the
lamina and- outwards along the groove which Ofiglnaily corresponds with the course
of the midrib, it tends more and more to cause elevation and divergence of the secondary
petiole and expansion of the lamina; and when the tissue has attained sufficient
.structural power to resist the older masses of tissue making for depression and convergence
of the secondary petiole and folding upwards of the halves of the lamina, a eondition
of permanent equilibrium is once more estabKshed. Until this has occurred, however,
movements necessarily continue to take place, because the fluctuations in turgescence
which accompany the presence or absence of solar stimulation in the younger and more
recently developed masses of tissue are greater than those taking place in the older ones.
So long as this is the case, the younger tissue is relatively stronger during the day than it
is at night, and consequently it is able to effect displacements diurnally which it is incapable
of mahitaining nocturnally. The magnitude of the movements goes on steadily increasing
so long as the differences between the functional and structural strengtlis of opposed
masses continue to increase; but, when the development of the older mass has once
been completed, the movements diminisK with the continued development of the
younger mass, and they uhiniately disappear when the tissue in it has overtaken that o£
its opponent in respect to structural strength and functional weakness. At the outset the
tissue -which makes for the diurnal position is present in very small amount as com-
]mred with that which makes for the nocturnal one, and consequently the amount of
displacement which it is able to effect is veiy limited. Subsequently it increases rapidly
in hulk, but at the same time the permanent structural power of its opponent has also
undergone increase. The mass of functionally stronger tissue increases, but the
strength of the structurally stronger one does so also up to a certain point. The
increased bulk of the functionally stronger tissue enables it to give rise to increased
displacement under the influence of solar stimulation; but so long as the differences in
permanent structural strength of the two masses go on increasing, the amount of resistance
which the structurally stronger one is capable of overcoming in the absence of
solar stimulation will undergo increase also. In consequence of tliis, we find that
nocturnal displacement does not attain its maximum in pinncc which have just begun
to move, but goes on steadUy increasing for a considerable time. The precise amount
of movement exhibited by any particular pinna at a given period in the course of
its development depends not merely on the relative bulks of the opposing masses of
pulvinar tissue, but on the differences which they present in regard to functional and
structural strength. The amount of diurnal displacement is at first small owing to
the small hulk of the tissue giving rise to it, and the noctm-nal displacement does
not attain its maxinium at first, because the mass of tissue which determines it goes on
for some time increasing in structural strength in relation to its opponent. Just before
the primary initiation of movement ia a pinna the supero-external mass of pulvinar
parenchyma is already conspicuous, whilst its opponent is practically absent: in a mature
or motionless pinna the condition is more or less reversed, the mass of the axillary tissue
being somewhat in excess of that of the external one. Owing to the fact that all the
pinnaj in a leaf are not matured simultaneously, but progresi.*ivcly from the base to the
. ROY. B o r . GAUD. CALCUTTA YOL. Y I.