THE CAÜSIS OP FLDCTBATIOKS IN TUSEBSOEaCE
preparations alone are studied, there would appear to be little structural ground ior
unstahlity of position in the tertiary puWni : in fixed ones the structm-al diflereuees
between the opposing masses of tissue are strikingly conspicuous.
The sti-uctural peculiarities of the tertiary pulvini thus present maximal facilities
for the occurrence of Tertical displacement. There is an extremely flattened fibrojasoular
bundle, and it is related above and below to large masses of tissue diffei-ini
from one another in the highest degree in regard to their functional and strnctnral
strength. Ihe tissue of the inferior pai-enchjma presents characters which imply the
presence of very considerable and continuous structural strength and of relatively little
habihty to fanctnmal altei-ations in tm^gidity. So long as water is suppKod to it
the collenchyniatons materials in the cell-walls will secure absorption, and conséquent
maintenance of continuous structural resistance in high degi-ee, whilst the scanty
protoplasmic content of the cell-caTÍties is little calculated to gira rise to extensive
t^uctuations ni active functional turgescence. On the occurrence of any sudden
diminution^ or interruption of water-supply, no suddeu diminution in the strength of
the tissue IS lilely to occnr, beeause the fluid which is stored up within the substance of
the cellenchymatous cell-walls cannot be rapidly removed, and, so long as it is present
It opposes an obstruction to tho filtrative escapo of any fluid from the interior of the
cell-cavities, both as the result of the great tliickness which it confers upon the cell
. a l l s and the rigidity of texture which its presence implies. In the inferior parenchyma
of the tertiaiy pulvini, we have to deal with a tissue providing not merely
tor active turgescence of the protoplasts of the cell-cavities, but for very • considerable
torgescence of the cell-walls. The former is a condition which is at once liable to
be aSected by fluctnations ,n assimüatory activity; but the latter bears no direct
relation to it, and will remain nnaltered so long as suflîcient water-supply is present
It IS not, therefore, aflected by the absence or presence of assimilate " L i v i t y T nd
remains as a constant feature of structural strength both diurnally and noctuiiially. On
the other hand the tissue of the superior parenchyma, with its relatively ooen texture
Its veiy delicate eell-walls, and its abundant protoplasm, provides the conditions foi
great fluctaafons in assimilatory turgescence, and also for the occurrence of processes
of rapid filti^tive loss. I, is constantly opposed by a mass of parenchyma'of very
mnch greater structm-al strength, and by the inherent tendency of the fibro-vascukr
bundle to resume .ts origmal position in relation to the rachis, and it is only under
circumstances which can its excess in functional strength into play and provide the
requisite supply of fluid to sat.sfy the osmotic capacities which are developed in the
cell-sap as the result of assimflatory activity, that it is capable of overcoming the
resistance to which it IS exposed. Any depression in general assimilatory a L i ty
must weaken It m the sti-uggle which it has to maintain, because its s t L g t h I
mainly dependent on active protoplasmic turgescence, whereas that of its opponent is
predominantly owing to mere physical properties of the cell-walls. Bnt it must also
necessarily be weakened under the incidence of any influences giving rise to decrease
m the general supply of hquid or favouring filtrative loss because oí the strnctaral
facilities which it provides for redistribution of liquid in its relatively open texture
and the debcacy of the walls of its component cells. The structural peculiarities of
the tertiary pulvmi as motor apparatus are thus such as to secure the occin-renoe of
regular periodic movements of a strictly nyclitropic character, and also to provide for
l a THE MOTOE ORGANS OP LEAVES. 9.:J
the occurrence of both gradual and rapid movements under the influence of any factors
affecting supply and loss of water generally, or calculated to give rise to alterations i„
local iiltrative piocesses.
There can be no question that the motor organs of Mim«a puH,« present structural
peculiarities which are of a nature specially to favour the occurrence of movements
under the influence of conditions of alterations in turgescence, whether dependent on
fluctuations in assimilatory activity, or on variations in conditions aftectmo- general or
local supply and loss of fluid, and that the capacity for rapid movement" which they
severally exhibit bears a direct relation to the textnral facilities which they individually
ofler for rapid filtrative redistribution. In the case of the primary pulvinus, very
exceptional provision for the occurrence of rapid redistribution of fluid is presented by
the deeper stratum of parenchyma, composed as it is of extremely open tissue, built up
of elaborately and extensively pitted cells, and in greater part struggling against ^rcat
resistance, both local and connected with distal leverage. In the tertiary pulvini, too,
similar facilities are present owing to the excessive delicacy and loose texture of the
superior parenchyma which has to contend with the overpowering structural strength of
the inferior one and the passive direction of tho fibre-vascular bundle; and in both
primaiy and tertiary pulvmi the capacity for rapid movement reaches a maximal
development. In the secondary pulvini, on tho other hand, the nature and relatively uniform
character of the tissue implies an absence of any structural peculiarities favouring
rapid redistribution of liquid in such excessively high degi-ee, and with this we find
capacity for rapid movements very much less pronounced. A study of tho motor oro-ans
in Minwm pudua alone thus leads to the same conclusion as that arrived at from°the
comparative study of the motor organs of nyctitropio leaves generally, that rate of
movement is ontii-ely regulated by structm'al facihties afi'ecting filtrative escape from tho
interior of the cell cavities of the tissues, whilst amount of movement is dependent on the
extent of possible fluctuations in tnrgescence in opposing masses of tissue. It alone is
enough to show how erroneous the idea is that, in the opposing masses of tissue in
pulvini, we have to deal with differenees depending solely on invisible molecular stiucturc,
and not on the presence of any visible ditieienoes in organisation.* '
CHAPTER XL
'Slicovits Hgarting tlu moticnunts of ike IcaDcs o{ ¿Jiimos« pniia.
In the preceding chapter an attempt has been made to demonstrate that the move
ments which are conducted by the leaves of Mimom f,dica are essentially the same in
nature and origin as those which are conducted by nyctitropic loaves generally, and
that any peculiarities which they present are to be ascribed to structural features in the
motor organs permitting of exceptionally rapid redistributions of fluid throughout the
tissues. This view is, however, entirely opposed to ordinarily accepted belief; for
although the regular periodic movements are commonly allowed to owe tlieh- origin to
the same causes as those which deteimino the occurrence of similar movements in other
nyctitropic leaves, the sudden movements arising under other cfrcnmstances than those
" l i s bandelt sicli nlso nucli liier irieder iiiuM urn sichtbare Organisationa Terliiillmssi
erkiati. sondtrn un, die unsiciubaie Meiekulstrulitur." Saclis, Verlesung S S 5 VI, a. 770.
13 deneu aieii die BeiabarJieit