^ ^ THE CAUSES OF FLUCTUATIONS IN TUBGKSCENCE
in tins case nlso is ono wliich is principally maintained bet-ween the fibro-vasculnr tissue
and the axillary pai-euchyina, the external pareuehymn from its unstable foundation being
lucapablo of making actively for convergence to any considerable extent. There is
therefore no cUfficulty in seeing why rise in turgekenee in the pulvinar pnrenchjTna,
ulthougli it be faii-ly unifoi-mly distributed, should be followed by movements of divergence
of very considerable magnitude; and there are structm-al reasons to account for
the fact that, as a rule, movements of any kind should be conducted only gradually. •
Both in their structural features, and in the chai-actors of the movements which they give
rise to, the secondary pulvini of Mimosa pudica much more closcly resemble the pulvini
of common wyctitropic leaves than either the primary or tertiary pulvini do. The
amount of tissue favouring exceptionally rapid rcdistributioD of fluid wliich they contain
is tritling in comparison to that present in the primary pulvini, and it is not uoi-mally
exposed to nearly so much resistance as it is in these. In the primary pulvini there is an
abundance of tissue liable to exceptionally i^pid filtrative loss in turgescence, and struggling
with excessive resistance dependent on inti-insio pulvinar cojjditious and distal leverage.
In the secoudary pulvini both the amormt of such tissue and the resistance to
wliich it is exposed are very much less, so that it would be strange indeed were the
capacity for rapid movement alike in the two cases. So long as the prunary petiole is
not deeply depressed, leverage does not come into play in making for convei-gence of the
sccondaiy rachises, and with this we fi-equently find sudden movements of them almost
entii-ely absent, and, as a mle, at the utmost very limited; but when deep depression of
the primary petiole docs occm-, distal leverage begins to act, and a correspondingly
increased tendency to the occurrence of rapid and considerable convergence of tho
secondary rauhise.s manifests itself under circumstances leading to decreased turgesciince
iti the secondary pulvini.
The tertiary pulvinus differs as strikingly in structural features from the secondary
one as the latter do from the primary. The entire organ is greatly compressed
vertically so as to present an elongated elliptical figure, and its fibro-vascular bundle
has the fonn of a broad flattened strip traversing the centre of the parenchyma
(Plate VI, Fig. 13). The following are the measurements of the various parts of
the section fi-om which the figure was obtained:—
Trausvme diameter of entire pulvinus ,.. ... 0'735 m.m.
TMekness of parenchyma to the left of the fibro-vaseular bundle ... 0-12 „
Breadth of fitro-vascular bundle ,., ... 0'48
Thickness of parenchyma to the right of the fibro-vascular bundle... 0'135
Vertical diameter of entire pidvinus ... ... 0-39
i> » of superior parenchyroa ... ... 0-18 „
„ „ of fibro-vascular bundle .., ... 0-06
!> » of inferior parenchyma ... ... O'lo „
Thickness of cell-walls of inferior parenchyma ... ... 0'0054 „
!. » » of superior „ ... ... 0-0009 „
I t is evident that the extreme vertical compression of both tlio entire pulvinus
and of the fibro-vascular bundle provide even greater facilities for vortical flexion
than are present in the primary pulvinus. The organ has not, however, to struggle
with any excessive disfal leverage as the primary pulvinus Las, and practically we
may regard its position as determined simply by the conflicting forces of the inferior
IN THE MOTOR OLLGAXS OP LEAVES. 91
and superior masses of parenchyma and the normal passive direction of the fibrovascular
buadle to tho fibro-vascular tissue of the secondary rachis.
Tho fibro-vascular bundles issue obliquely from the bevelled surfaces on either
side of the upper half of the secondary rachis, and arc originally directed forwards
and slightly upwards from their sites of origin, and horizontally parallel to the lino of
the rachis (Plato V, Fig. 5). Owing to the obliquity of origin of each bundle its upper
surface looks upwards and forwards, and its lower one dowuwai'ds and backwards so long
88 the original position is maintained or resumed. Tho masses of parenchyma lying
above and below the fibro-vascular band differ strikingly from one another in stnicture,
and are sharply defined from one another even where they are not separated by tho
bundle (Plate VI, Fig. 13). The superior mass is shghtly thicker than its opponent.
I t s component cells have very thin walls and relatively abundant protoplasmic contents,
and the tissue is comparatively an open one containing numerous, large, intarcellular
spaces ill addition to minute angular ones. The cells of the inferior parenchyma
have walls five or six times as thick as those of the superior one (Plate VI,
Figs. 14—17), the tissue which they compose is a very dense one, and the amount
of protoplasm which they contain is relatively trifling in amount. Tho thii'kness
of tho cell-walls in the superior parenchyma is aobut -0009 m.m., that of the inferior
one about -0054 m.m. There is not any considerable difference in tho amount
of chlorophyll content of the two masses, but the upper one when in a condition of
compression, as it ordinarily is in the case of detached pulvini, appears greener than
its opponent.
The appearances described above can be cleariy made out in fresh sections in
water or acetate of potash, but are much less distinct ia permanently mounted sections,
unless these have been fixed by treatment with osmic acid ere exposing them to the
action of other reagents. In order to obtain permanent preparations representing the
tissue as far as possible in a natural condition of full turgidity, the freshly
cut sections ought to be allowed to be in water exposed to light for some time, so
as to allow conditions of turgidity under solar stimulation to be re-established, and
then immersed for a fow seconds in a 2 per cent, solution of osmic acid. This
treatment secm'es two t h i n g s — f i x a t i o n of the cell-walls to some extent, and second
fixation of the protoplasmic contents of the cell-cavities. Unless it be adopted, the
tissue of the superior pad will ccrtainly be more or less shrivelled and collapsed, and
t h e coil-walls of the inferior ono will lose the greater part of their thickness. The most
satisfactory permanent preparations are those which, after being stained with pm-ocarmino.
are mounted iu Farrant's solution; but even those mounted in balsam are much more
)-eprcsontative of tho natural condition of the tissues than any mounted without fixing.
The greater part of the tliickncss of the coll-walls in the inferior pai-onchyma is
manifestly composed of materials resembliag those in the thickened areas in the walls
of common colleuchymatous tissues, and therefore normally subject to great reduction
in bulk on exposure to conditions leading to extraction of water. In unfixed sections
which are subsequently exposed to immersion in absolute alcohol, &c., prior to mounting
in balsam, a complete extracliou of water, of coiu-se, occui's, and with this an excessive
reduction in the thickness of the cell-waUs, and the same effect in somewhat minor
degree occurs in unfixed sections mounted in Farrant's solution; but treatment with
oamic acid allows of extraction of water witliout diminution in volume. If unfixed
ANN. BOY, BOT. GARIV CALCUTTA VOL. V I.