73 THE CAUSES OF FLUCTUATIONS IN TUBGESCENCE
amount of rotation, seeing that at an early stage of development the upper surface
of the primary rachis faces directly inwards to the axis, just as it does in the case
of Pilhecohhium saman. Where the growing shoots ai-e either erect or more or less ascending,
iis they are in FWteculohium saman, simple divergence, or divergence accompanied b y a limited
amount of rotation of the primary petiole, will suffice to cause the upper surfaces of
the primary racliises to face directly upwards; but where the axis is more or less
horizontal, it is clear that simple divergence could only produce such an efiect in
the case of petioles which originally faced directly downwards, and then only when
carried out in vciy excessive degree, and that in all other cases rotation (and
frequently excessive rotation) must necessarily come into play. The ultimate
position of the leaves is not, however, entirely due to rotation of the petioles,
but also to the occun-euco of a certain amount of spii'al movement in the
gro-^'ing axis, wliich in the course of development undergoes a sort of uncoiling,
so that e.vcli successive leaf at a certain period comes, to be more or less latei-ally
situatsd. A mass of pulvinar parenchyma is gradually developed over the outer face of
the base of the petiole. The accumulation of tissue is not, however, unifoi-mly distributed,
but predominates on that side which is for the time being the inferior one;
and with this, rotation of the petiole gradually sets in and advauces through a
quadrant, so that the oiiginally inferior aspect comes to be axillary, and its structurally
axillary one comes to look directly upwards. But, owing to this rotation, the predominant
mass of pulvinar parenchyma which was originally inferior necessarily assumes an
axillary position, and conssquently makes for divergence in place of lateral elevation
of the petiole. In the matui-e leaves a great mass of axillary pulvinar parenchyma
is very conspicuous; this, however, does not belong to the structurally axillary face of
the petiole, but to that lateral aspect of it wluch origraally wan dii-ected downwards.
The extreme inequality in the development of pulvinar tissue in respect to the two
sides of the vascular axis of the pulvinus, together with the weakness of the latter,
is illustrated in Plate HI, Figs. 5, 6.
Wliilst periodic movements are relatively inconspicuous in the primary petioles, they
are Mgbly developed and very persistent in the case of the secondary rachises and
mule's. As in other instances, they ai-e prnnarily absent; but from the time at
pii
which they appear, they normally continue to be carried out M-ith unimpaired vigour
during the entire peiind in which the leaves retain a healthy character. In this
respect the leaves resemble those of Mimosa pudiea- much more closely than do any of
the leaves which have liitherto been considered. Before the period at which periodic
movements set in, the secondary petioles are primarily directed upwards and forwards
from the line of the primary one, and ai-o so closely convergent as to have tlioii- inner
sm-faces in contact with one another (Plate I I I , Fig. 7). Somewhat later they bccome
somewhat divergent wlulst still retaining their ascending direction (Plate I I I , Fig. 7). IJoth
divergence and elevation next undergo progres.sive diminution; and, immediately prior to the
onset° of periodic movements, the secondary petioles are once more closely convergent
to the line of the primary one, and lie in the same plane with it or slope sligiitly
downwards fi'om it. When periodic movements do make theii' appearance, they give
rise to alternating nocturnal convergence and depression and diurnal divergence and
elevation to the plane of the petiole. The pinnules, like those of iHihecolohium
saman and 3imosa pudiea, are from the outset fully expanded and, until the onset
of periodic movements, are directed forwards iu a hne parallel with that of the
Hi THE MOTOU ORGANS OF LEAVES.
secondary petiole, with the upper surfaces of tli'ise of the opposite rows in close
contact with one another, and their surfaces facing dii-ectly outwards, the outer or
posterior half of the lamina3 lying vertically over the axillary half, and the midrib
})oiiiliug directly forwiu'ds. When movements set in they lead to progressively
increasing diurnal divergence, rotation and depression of the lamiiia3 which ultimately
come to lie in the same plane with the upper surface of the secondary petiole, and
more or less at right angles to it, with tlieir upper surfaces facing dh'ectly upwards,
alternating with a nocturnal resumption of the originally perniiinent position, or
rather of that position slightly modified by the fact thnt the midribs now, as a rule, no
longei' point directly forwards, but slightly downward.? from the line of tlie secondarjpetiole.
The movements of the pinnules thus differ from those in the pinnules of
PilhccoloUum saman, and are of the same nature as those in the pinnfc of Cassia alaia
in consisting not niei'ely of alternating elevation and depression, but of alternating
elevation and depression combined with a considemble amount of rotation.
The stiuclural features which detennine the character of the mov ements of the secondary
petioles are of the following nature. When an actumulalion of pulvhiar parenchyma
begins to be developed, it appears first over the upper half of the base of the secondai y
petiole; and, together with the increasing leverage of the distal parts, tends to give rise
to gradual depression. At a somewhat later period accumulation begins over the lower
half, imd this of course makes directly for elevation. In mature but still Jreely motile
pulvini, there is little difference in the thickness of the coll-walls in the various portions
of the parenchyma; but the mass which lies to the axillary side of the axis is distinguished
from the outer one by greater thickness (Plate I I I , Fig. 9), and hy containing a
greater number of green chromatophores. It is this, together with the fact tliat the
axillary masses on undergoing any increase in turgescence press upon the resistent mchis
in the case of alb the pinnje but the terminal ones, and against one another in the
latter, that detemincs the gi-eat diurnal divergence of the secondary petioles. The
external mass of pulvinar parenchyma is less bulky and poorer in chlorophyll, and
consequently undergoes less increase in turgescence diimially than its opponent ; and,
moreover, it is related basally to a mass of spongy tissue very poor in chlorophyllcontent
(Plate I I I , Fig. 9), so that any rise in turgescence wliich it does undergo is in
great measm-e expended in compressing tliis feeble tissue. With the absence of any
conspicuous stmctural differences between the superior and inferior parenchyma, and the
presence of a relatively strong vascular axis (Plate III, Figs. 8, 9), which presents very
considerable resistance to both vertical and horizontal flexion, we iind the amount of vertical
movements becoming very inconsiderable. In Pithecohhium the secondary rachises in
leaves at the stage in wliich movements are at a maximum are nocturaally directed backwards
at acute angles to the line of the primary one; but in Leuccena they never
attain to a right angle to it, but are invariably dii-ected forwards at very acute angles
towards its apex. The really impoi-tant struggle is that wliich is carried on between
tlie strong vascular axis of the pulvinus and the mass of parenchyma lying to its
axillary side, and which is diurnally able to divert it from its normal passive direction.
'J'he axillary parenchyma, fronr its pos^ition, its great«- bulk, and its greater- richness in
chlorophyll, is able not only to do away with any effects of diurnal rise in turgescence
in the external pad, but also to cause extreme divergence of the resistent vascular axis.
In the case of highly developed pulvini the vascular axis is reduced to the form of a
solid cord containing no medullary tissue. Where tliis has a cylindrical foi-m, as in the
. ROY. BOT. GAUD, CAI - V I .