B
98 TOE CÍÜSE3 OF FLITCTUATIOKS IN TURGESCENCE
pulvini of Mimosa pjidlca, very considoraWe alterations in relative stveiigtli must tlius
be rapidly ostalili^icd and, the eciiiilibrium of position being correspondingly disturbed,
JUOTements must occur with proportionate rapidity.
But if all the movements of the leaves of Mimosa pudica, apart from those whicli
attend the presence or absence of solar stinnilation or other agcneies afiocting assiiniiatory
activity, owe their origin to such purely physical causes, the readiness and the
degree in which they manifest themselves onglit certainly to bear some evident relation
to the presence or absence of conditions favouring tlie occuiTonce of rapid alterations
in the relations existing between general loss and supply of iluid, or affecting the
activity of processes of local filtration; and there is abundant evidence that such a
relation really exists. Its existence is demonstrated by the fact that the occurrence, rate,
and extent of movement following the incidence of various disturbing agencies are alike
manifestly affected by conditions of telluric and atmospheric humidity- According to
Haberkndt's ttieory, the extent of propagation of movement ought to bear a direct
relation to conditions securing excessive root-supply and depressed transpiratoiy loss;* but
in fact the relation, in placo of being a direct, is an inverse one. When the soil is
saturated and atmospheric humidity excessive, the capacity for niovements and the rate
and extension with which they occur are alike at a minimum, and it is when the soil is
relatively dry and atmosphei-ic humidity low that they attain their maximal development.
I t would be strange indeed wore this not so, seeing that conditions of atmospheric and
telluric saturation are calculated to increase the stability of turgescence, whilst aridity
of soil and air must inevitably tend to render it unstable. So long as air and soil are
alike saturated, there can be nothing to interfere with the tissues fully satisfying any
osmotic capacities which they may possess ; and consequently, apart from variations in
the degree with wJiich they exercise their assiniilatory functions, tliere is nothing to
interfere with the continuous maintenance of turgescence at a given standard. All the
tissues may not possess osmotic capacity in like degree, but the conditions are such as to
allow them all fully to satisfy what capacity they have. There is an entire absence of
any struggle between osmotic capacity and any agencies tending to give rise to genera!
loss of water. The only loss of water which is going on is that connected with assimilatory
decomposition, and this is amply covered by the excessive root-snpply. But when
the soil and air are not saturated, the equilibrium of turgescence can only be maintained
so long as general supply and loss of water continue to bear a unifona relation to one
another. Any rise in loss without corresponding increase in supply must make for decreased
turgescence, and any fall in loss without corresponding depression of supply for
increased turgescence. A high degree of turgescence may in certain cases coincide with
a gj-eat capacity for movement, but only when it is maintained in spite of rapid
general loss of water ; only when it exists as the result of an accurate balance between
supply and loss; and therefore .we do not find the capacity for movement high in cases
•where conditions are such as to give rise to diminished transpiration without affecting
assimilatory activity and therefore osmotic property.
I t would be easy to multiply examples of experimental results showing the truth of
the statements contained in the previous paragraph ; hut in order to avoid breaking the
continuity of the text to an inconvenient extent, one or two selected examples alone are
introduced here, and the mass of experimental evidence is transferred to the tables which
* Op. cit.
IN THE MOTOE OKGANS OF LEAVES. 99
are contained in Appendix E. The effects of exposure to the influence of saturated soil
and atmosphere in diminishing capacity for movement aro constantly presenting themselves
in the most conspicuous fashion in various ways. "Whilst working at the subject
I have had constant occasion to take specimens of Mimosa jmdica from the garden
in which they are grown to my laboratory which is situated at a distance of about a
mile from it. Whea the transfer is effected during periods when atmospheric humidity is
relatively low, even slight displacement of pot plants is followed by extensive movement
of the leaves, and the agitation to which they are exposed during the drive over the
rough road which intervenes between the two localities certainly leads to maximal
movements in the pinnules and primary petioles, and to considerable convergence of
the secondary rachises. But with increased humidity the tendency to the occurrence
of movement decreases; and when humidity of soil and atmosphere are at a maximum,
the plants undergo all the agitation to which the ti-ansfer exposes them cither without
exliibiting any movements at all, or movements of only the most limited and
partial kind. Parallel phenomena make their appearance under the influence of artificial
elevation or depression of atmospheric humidity. If two plants luted into carefully
stoppered and waxed pots be exposed side by side in two hermetically sealed chambers
one containing a vessel of water and tlie other a vessel of pure sulplmric acid, their
leaves are of course coincidently exposed respectively to a saturated and to a very dry
atmosphere. As the result, transpiratory loss is ai-rested in the on© but rises to a high
level in the other, and corresponding differences in their so-called ii-ritubility also make
their appcarance. The leaves in the saturated atmosphere become highly elevated and
extremely expanded, and after an hour or two of exposure they fail to respond even to
the violent agitation produced by shaking the chamber within which they are enclosed.
The leaves in the sulphuric acid-chamber, on the other hand, also show extreme
expansion and elevation, but are very susceptible to agitation, even slight shaking of
the chamber being followed by depression of the primary petioles and complete
assumption of the nocturnal position by the pinnules. So, again, when a leaf or a
portion of a shoot bearing several leaves is separated from a plant at a time
when ail- and soil are alike relatively dry, the separation is followed by immediate,
rapid and complete movements in the pinnules and primary petioles, and by
considerable convergence of the secondary rachises; whilst when atmospheric and telluric
moisture is excessive, no movements whatever n)anifest themselves for some time, and
the movements which ultimately do occur ai-e very slowly, imperfectly, and gradually conducted.
Under conditions of maximal moistm-e, the pri.nary absence of any movement
whatever, and the delayed and imperfoet character of those which ultimately occur,
bccoiiie woiidorfiilly conspicuous. Under such circumstances, shoots will frequently stand
exposure to all the disturbing influences of separation from the axis, and of considerable
and relatively prolonged agitation without showing any appreciable movements. I have
accordhigly been able to separate shoots and transfer them from the siie in which they
wore grown to my laboratoiy without their showing any more evidences of movement
than rooted plants do under conditions of iiigli soil and atmospheric mnistm-e.
Under similar circumstances exposure to conditions which determine the ai-tificial
escape of liquid from the tissues produce minimal effects as regards movement. When
the air and soil are both moderately dry, amputation of the tip of a distal pinnule is
normally followed by immediate and rapid action in the injured pinnule and simultaneous,
or practically simultaneous, action in the opposite one, rapid spreading centripetal
. R o ï . BCT. GARD. CALCUTTA TOI,. T I.