66 THE CAUSES OP FLUCTUATIOi;S li! TDBGESCENCE
apox of the rachis, the cliiu-iial and uoctunial positions of all the indiyickal plnnrc
in one and the same leaf are nob uniform during the greater part of the period
during which appreciable movements are present. This is cleai-ly illustrated by the
following notes regarding the noctinnal positions of the pinnee in a single leaf. All o£ tlie
pinttaj ^Yel•e closcly convergent to the line of the petiole and depressed beneath it. In the
two terminal pairs depression was not so great as in the succeeding ones, and the halves
of the lamina were somewhat folded upwards. In the next four pairs depression and
convergence were extreme, and the laminic were fully expanded, and had their upper
surfaces facing directly forwards to the tip of the leaf. In the nest pair the upper
surfaces of the lamióte were slightly inclined inwards, and in t!ic succeeding three
pairs they faced almost directly inwards. In the two basal paii-s depression and
rotation were ah-endy somewhat diminished, and the upper surfaces of the laminje were
consequently directed forwai-ds, upwards, and inwards. In this leaf the distal pairs of
pinna? had not yet attained to the maximum of movement, wliilst tlie basal ones had
already begun to decline from it.
The phenomena of movement in the leaves of Casaia Suma'rana indicate just as
clearly as those of Cassia alala that they correspond with the interposition of a period
of instability between two periods of stable equilibrium in the tissues of the motor
organs, and that, so long as movements continue to occur, the assumption of the nocturnal
position is always owing to the action of a tendency to tlie resumption of what was
either the primary permanent position or tlie diurnal position at an earlier stage in
the evolution of tlie pinna?. In the initial periods of movement, the nocturnal
conver"-ence, depression and elevation of the halves of the lamina give rise to complete
reversion to the antecedent permanent position. Somewhat later rotation comes in to
interfere with the completeness of the reversion and to replace folding of the lamina.
So long as folding is possible, the action of the portion of the mass of pidviuar
parenchyma making for the noctural position, which is prolonged into the under
surface of the lamina, assists in elevating the outer half of the lamina from
the plane of the midrib; but when permanent expansion has been established, folding
is no longer possible, and the same action tends to cause torsion of the midrib.
Rotation is, however, a comparatively transitory phenomenon here as compared with
the rotation of the pinnie of Cassia ala/a, because there is no excessive accumulation
of pulvinar parenchj-ma on the iinder surface of the outer half of the lamina, so
that the gi-owth of the oppof-ing tissue on the other side of the pulvinus comparatively
"rapidly provides sufficient resistance to prevent the occurrence of torsion
of the midrib and vascular axis of the pulvinus. Rotation is thus present only during
the period at wHch the mass of pulvinar tissue making for the noctural position
possesses its maximal relative strength.
In Cassia Siimatrana, as in Cassia alata, the movements of the leaves, although of
very considerable magnitude when at thc-ii- maximum, are invariably slow and gradual.
In both cases the tissues of the motor organs do not provide any special tíltiíitive
faciHties, either in the foi-m of abundant pitting of the cell-walls or of an extensivo
system 'of intercellular spaces (Plate VII, iMg. '¿), and hence any factors wliich tend
to give rise to sudden increments in external pressure or^ to sudden increase in
general loss or decrease in general supply of fluid, are incapable of producing
any immediately appeciable effects on turgescencc.
IN THE aiOTOtt ORGANS OP LEAVES.
CHAPTER IX.
%ht inobcmtnts o£ lite lenljcs of ^itlxecolobimn pitman iinD
g e u c a i i i i i glaiicit.
The movements of the leaves of FithecoloUum sanian arc distinguished from those
occurang in the leaves of either of the species of Cassia Avhich have been treated of m
the previous pages by much greater persistence and by the relative rai)idity with which
they are executed. The movements of the primary petioles, although originally very
conspicuous, do undergo rapid diminution at a comparatively early period in the life
of tlie leaves, and are practically absent latterly; but those of the secondary petioles
and pinnules are very much more persistent, and it is only in very old leaves that
they become inconspicuous. "When the movements are at a maximmn, the diurnal and
nocturnal positions are of the follomng nature :—diurnally, the primary petiole and
rachis are very widely divergent from the axis, l)ang almost at right angles to i t ; the
secondary petioles and rachises are also very widely divergent from, and He almost in the
same horizontal plane as the primary ones, and the pinnules are in the same plane and
have theii- midribs diverging from tlie line of the secondary racliises at angles of
rather more than 45 degrees. Noctvu-nally, the primary petiole is so highly convergent
that it not mifrequently actually crosses the line of the axis ; the secondary petioles
ai'e converged and so deeply depressed that they come to point more or less backward
towards the base of the leaf (Plate VII, Fig. 7); the pinnules are folded almost directly
upwards, so that the upper surfaces of those of the opposite sides of the secondary
rachises are in close contact, theii' midiibs being directod upwards and forwards at
vertical angle.s with the line of the secondary rachises of almost the same degree of
inclination as that of the diurnal horizontal one; in other words, they have undergone
almost pure elevation apart from any appreciable rotation.
When dealing -with the subject of nyctitropic niovemeuts generally, it has ah-eady
been pointed out that the nocturnal position in tliis case is specially clearly one of
reversion to what was the pemanent one at a period immediately preceding the
time at which periodic movements first begin to make their appearance, and that the
diurnal divergence of the primary petiole is essentially connected with the gradual
development of a great mass of axillary pulvinar parenchj-ma. The action of the
dim-nal rise in turgescence in the primary piilviniis must, however, in this case be
greatly aided in effecting ilivergence by the coincident increase in tm-gescenco and alteralioa
ill the position of the distal portions of the leaf, wliicli imply corresponding increase
in distal leverage. The leverage here aids in establishing the diuraal position, and renders
it. more stable than it would othei-wise be, or than it is in leaves like those of Mimosa
pudica, in which distal leverage opposes, in place. of aiding, tlie tissue in the primary
pulvinus wliich makes for the dim-nal position; and to it, no doubt, to a great extent
is due the phonomeuou of rapid diminution and early disappearance of rhovement in
the primary petiole.' Large leaves are provided with from six to eight secondary
rachises which bear from tlu-eo to eight or nine pairs of pinnules, the number of
the latter being smallest in the basal rachises and increasing progressively towards
ANN ROY. BOT. GARD. CALCUITA YOL. V I .