r-1 THE CAUSIS OF ILüCIUilIOXS I» TtJEGlSCESCI!
secondary pulvini of Mimosa jmiiea, tho masses of pulviiiar parciidiyraa surromiding it
either eq.ml or exceed it in tliickncss; Tihoreas in tire secondary piih-ini of FiihcMmm
mmnn it takes the form of a Battened hand, tho Biipeiior and infeiior masses csceed
it in vei-tical diameter, and tho lateral ones falling short of it in horizontal diameter; hnt
of I m the secondary pnlvi m ¡lauca the tliickncss of the asial mass of yasonlar
and mednllary tissue considerably exceeds that of the Ending lasses of pirlvinar
parenchyma in every direction. (Wate I I I , Figs. 8, 9.)
In the case of the tertiary palvini the opposing masses of pulrinar parenchyma arc
disposed aboTe and beneath tho vascnlar axis, but the moTOinonts wHch they determine
are not simple moYcments of depression and olcTation, like those present in tho pinnules
of FUhccMium laman. This is omng to the following facts. In tho first place,
the pnli-ini are obliquely set on upon a boTclled surface, so that the opposed masses
directed respectively upwards and forwards
they ai-e in the tertiary pulvini
second place, they hold
that wliich is present
tion of the opposing p u h ™ . - ^ . , . .
not merely for depression, but also for backward displacement of the lanima, and tho
under one not merely for elevation, but for elevation and advance. So far, of course
wo are dcaliu"- with movements of depression and elevation on an oblique sm-face, but
tho character of the movement is further modified by the unlike relation which the masses
of pulviiar parenchyma bear to the two halves of the lamina. There are conspicuous
ereansions of pulvinar tissue involving tho base of the posterior or outer half of tho
lamina both on its superior and infeiior aspects (Plato III, Fig. 10), and these, of
course, must tend respectively to determine rotation outwards and inwards of tho
surface of the midrib accord
and downwards and backwards, just as
of 31imiii pudioa (Plato V, Fig. 5); and in the
' relation to the lamina of the pinnule vory similar to
1 tho case o£ tho pinnaj of Catsk, alata. The obhquo disposi-
• masses necessarily imphes that the upper one must make
to the variations in theii relative strength
upper Butlace or mo - o
and resistance. The inferior pulvinar pad consequently not only makes for general
advance and elevation of tho lauiui, but for excessive elevation of its outer h»H,
and when once the displacement has advanced so far that tho posterior side of the
keel of the midrib comes to lie superior to the anterior side, to a deflection of tho pinnule
The ultimate outcome of the conditions is that when the leaf is in the feUy devel-
„ , „ d nocturnal position, tho pinnules not only have theii- upper surfaces facing directly inw'ards
but slope downwards and foi-wards fi'om the plane of tho petiole. An increase in the
r e l a t i 4 stren.'th of the upper mass of pulvinar parenchyma conversely not only gives
rise to depressive divergence and retrocession, hut also to special depre.ssion of the outer
halt of the lamhia, and consequently to rotation of the vascular axis, so that the upper
surface of tho midi'ib, in place of facing inwards, faces directly upwards, and the upper
surface of the lamina lies in the same plane with and mote or less at right angles to
the socondaiT rachis. There is not any vory conspicuoas difference in the strength and
thickness of the cell-walls of the opposing masses of tissue, but the superior ono is considerably
richer in chlorophyll coi-puscles than the mfonor one is. ^
The pnWnar tissues in leaca,m gluuca are oven more extensively and conspicuously
pitted than those of P i t t « « U ™ are: but they resomble tho latter and differ
tan those of the piimary pulvini of Minma fudica, in bemg throughout devoid
of a n r w g - y developed Tystem of intercellular spaces (Plato VII, Figs 3, 4 5).
i b i s S i c i i l y conspicuoii, in regard to the primary pulvini of tho deeper strata
w M in E . % - « are net merely vory extensively pitted, but are of extremely
IN THE MOTOR ORGA^TS OF LEAVES. "•'>
open texture, wliilsfc in PUhecolohiuin and Leucoena they contained merely minute
angular intercellular spaccs like those of the superficial ones. Col-responding with
these difierences in the degree to -which facilities for redistribution of fluid aa-e present,
we find differences in regard to the rate at which movements are executed. The
leaves of Leucoena move more rapidly than those of PUheaolohium, but not nearly so
rapidly as tliose of Mimosa. When the temperature is liigh and atmospheric humidity low,
a certain amount of depression of the secondary rachises and convergent elevation of
tlie pinnules makes its appearance almost immediately after a leaf or shoot has been
detachcd from the axis, and much sooner than any corresponding displacements are visible
in leaves of Pitheeohbium exposed to similar conditions. This is in one respect remarkable,
as the numbers of stomata in L'Ucoena are very much smaller than in Pilhecolohmm-
In spite of this, however, transpiratoiy loss takes place with very great rapidity, probably
in connection with the general delicacy of the tissues, as is indicated unequivocably
by the very early onset of phenomena of general wilting. As the atmospheric
humidity rises, the rate of movement falls, and when a condition approaching saturation
is present, movements are vory greatly retarded so long as mechanical disturbances
of the leaves calcalated to give rise to increased filtrative discharge of fluid
from tlie pulvinar tissues are absent. In Leucoena movements fending towards
the establishment of the noctm-nal position may, even more distinctly than in the
case of any of the leaves wliich have been previously described, be recognised as
arising under the influence of many different conditions which only agree with one
another in the fact that they all tend to induce diminished turgescence in the
pulvinar tissues. In cases of sepaj-ation of leaves or shoots from the axis, dimiuished
turgescence arises in consequence of arrested supply and continued transpii-atory loss
of fluid; whei-e movements occur in connection with continued exposure to hot diy
air without ssparation fi'om the axis, excess in transpiratory lo>s acts alone; where they
follow much meclianical agitation, increased local filtration in the pulvinar tissues come.s
into play; and, finally, where they attend removal of solar stimulation, a faU in
osmotic capacity of the cell-sap is the detei-minant. In all cases alike, the loss of
strength, or, in other words, the diminution in turgescence falls most heavily upon
the younger and functionally most active masses of tissue, and consequently, in all
alike, the movement which attends it is in the direction of the position winch is
made for by the older masses of pulvinar tissues, because they are the sti-ucturally
stronger and functionally weaker ones. In certain cases the sti-uggle is mainly between
opposing masses of pulvinar parenchyma, wliilst the vascular tissues are practically
passive; in others the vasciilai- tissues co-operate with the older masses of pulvinar
parenchyina ; and in still others they become the main determinants of the assumption
of the nocturnal position. In any case, however, the struggle is between functionally
powerful and structurally powerful tissues, the former constantly making for tlie
diurnal and tho lattci- for the nocturnal position. The tissues which are distinguished
by structural strength neither undergo so much diurnal increase in turgescence, nor
provide such facilities for filtrative loss of fluid, as those which are chai-acteiised by
excess ia functional strength; and consequently they are not subject to so much loss
of turgescence on removal of solar stimulation or on the incidence of conditions
calculated to give rise to increased filtration as the latter are; so that any deprivation
of light or increase in pressure on the pulvinar tissues must tend to give rise to an
assumption of. the nocturnal position. When wc compare the rates at wlii(;h movements
. E o ï . BOT. GAUD CALCITIA TOL. V I.