6 8 THE CAUSES OF FLUCTUATIONS IN TUEGESCENCE
the distal portions of the leaf. In the case of five large leaves in "which the actual
number of secondary rachiscs and pinnules was coimted, that of the former varied from
t-wolve to sixteen, and that of the latter from seventy-eight to one hundred and four!
which very conclusively indicates that the amount of leverage telling on^ the primary
puU-inus in favour- of divergence must be very considerable; and so considciable is it
that, in co-operation "with the gi-eat progressive development of the mass of axillary
parenchyma (Plate I, Fig. 14), it not unfrequently leads to the leaf ultimately becoming
permanently dii-ected more or less backwards towards the base of the axis. In the
case of the primary petiole and rachis there is a nocturnal reversion to a previously
permanent position, only slightly modified by a certain amount of increase in convergence
; but in tliat of the secondary ones considerable modification is present owing to the
fact that for some time the amount of nocturnal depression goes on steadily and
progressively increasing. The nocturnal position of the pinnules, on the other hand, is
one of pare reversion, and the alternate movements of diurnal depression and
nocturnal elevatioQ wliich they undergo are the exact parallels to the alternate unfolding
and folding of the laminie of young pinus of the species of Cassia. There is no
appreciable rotation similar to that occurring in the case of those pinnce, because
the opposing masses of palvinai- tissue making respectively for the diurnal and
nocturnal positions lie directly superior and inferior, and are related to the upper and
under surfaces of both halves of the lamina alike. The inferiorly situated mass of
parenchjTiia is aided in giving rise to nocturnal elevation of the pinnule by the
action of the vascular axis of the pulvinus, which constantly tends to resume its original
direction to the secondaiy petiole. It is thus only after it has attained considerable
magnitude that the superior mass of pulvinar parenchyma is able to overcome the
resistance of the inferior one and of the vascular bundle; and, even when it has
attained , its raaximuui development, it is, as a rule, incapable of securing complete depression
of the pinnule to the plane of the secondaiy petiole. The great relative
increase in bulk of the superior mass of pulvinar parenchyma as compared with the
inferior one, which takes place during maturation of the leaf, is illustrated in Figs.
16 and 17 of Plate I.
The opposed masses of parenchyma in the secondary pulvihi are essentially disposed
altogether superiorly and inferiorly; but yet the movements which actually occur
are not movements of simple depression and elevation, but of depression and convergence,
and of elevation and divergence. This is owing to the peculiar relation
wliich tlie pulvini bear to the bevelled petiolar sm-faces with which they are
connected (Plate IV, Fig. 5). The superior mass of pulvinar parenchyma, as the figure
shows, is so situated as to lie so much above the level of the, bevelled surface of the
primary racliis that any alterations in its relative strength and pressure can only
act appreciably in giving rise to movements of elevation and depression; but in the
inferior pad, wliich is internally in close contact with the bevelled surface, any
increase in its turgescence must tend not merely to give rise to elevation, but also to
divergence of the secondary petiole. In the normal passive position, as determined
by purely structural features, tlie axial vascular bundle of the secondary pulvinus Hcs
ahnost parallel with the line of the primary petiole; and any rise in turgescence in
the axillary portion of the inferior mass of pulvinar parenchyma must tend to cause divergence,
owing to the relation which the tissues of the racliis and pulvinus bear to one
another. The movements are not in this case complicated by the presence of any
IN THE MOTOR ORGANS OF LliAVES.
rotation Hke that which occurs in those of the secondaiy pulvini of the Cassia,
because they are not related to the presence of any structm-al airangcments corresponding
to the laminai- expansions of pulvinar tissue, which are present in the latter.
It is the very peculiar foi-m of the vascular axis in the secondary pulvini that is
the principal dorerminant of the extreme convergence and depression of the secondary
petioles during the night (Plato IV, Figs. 4-5). The vascular tissue, in place of forming a
more or less rounded cord, as it docs in the case of the secondary pulvini of Mimosa ptidtea,
or a tube filled with medulla as it does in those of Leiicana glauca, talies the form of a
broad, flattened, slightly-cui-vcd band. This naturally implies corresponding modification in
the resistance which it will present to horizontal and vortical displacement from its normal
passive position. It is morphologically adapted to afford very considerable resistance to
the divergence which is determined by the axillary portion of the inferior mass of pulvinar
parenchyma; and consequently, when the latter loses turgescence with the removal
of solai- stimulation, it acts powerfully in producing convergence. But its resistance to
depression is very feeble, and the diwtal leverage to which it is exposed is very great—
the pulvinus only weighs from 4 to 7 per cent, of the entire secondaiy petiole and pinnules
—and consequently when it loses the support afforded to it by the diurnal excess of
turgescence in the inferior mass of pulvinar parenchyma, it yields to the leverage aided
by the action of the relatively feeble superior pulvinar parenchyma. The central
portion of the inferior pulvinar pad is not only thicker, but also much richer in
chlorophyll, than that of the superior one is; and consequently, when in a condition of
diui-nal turgescence, it is able not only to overcome it, but to overcome the action
of distal leverage on tlie vertically flexible vascular axis of the pulvinus. The lateral
portions are not conspicuously thicker than those of the superior pad; but the inner
of them, when in a condition of diurnal turgescence is, from its axillary position,
enabled to effect divergent displacement of the vascular bundle in spite of the relatively
great resistance wliich it opposes to horizontal flexion. (Plate IV, Fig. 5.)
In the secondary and specially in the tertiary pulvini, during the period in the
life of the leaf in which extensive movements are regularly carried out, the masses of tissue
wliich make for the diurnal position are conspicuously distinguished by their colour
from those which make for the noctui-nal one. In both cuses they are of a deep
green, whilst theii- opponents have a much paler ochreous green tinge (Plate IV, Figs. 3,
5—9). Consequently, ia the case of the secondaiy pulvini, it is the inferior mass of
parenchyma which is green and the superior which is ochreous; whilst in the tertiary
ones precisely the reverse holds good.
The opposed masses of parenchyma in the primary pulvini do not show such conspicuous
and readily appreciable differences in colour as the corresponding tissues in the
secondary and tertiary pulvini do, but the steady and progressive increaee in relative bulk
which takes place in the superior one coincidently with diminution in and final
abolition of the assumption of the noctunial position by the primary petiole is very
sirildng. {Vide Plate 1, Figs. 14, 15; Plate IV, Figs. 1, 3.) In leaves in which petiolar
movements are at a maximum, the superior mass is distinguished merely by the
fact that it is vivid green almost throughout its entire thickness. Whilst, in the
inferior mass, the tissue nearest the wood is almost colourless, and the superficial
band of green has a somewhat olive tinge. Microscopic examination, however, shows
that the tissue of the superior mass is composed of cells wliich are of considerably
smaller size and much more richly provided with greea chromatophort'S than those