THE CAUSES OF FLL'CTITATIONS IiN' TUKGESCli;^'CE
(lisplaceinent Avliich actually oocurs v,-i\], however, be affected b}- the conditions of
g-eucral loss and sui)ply of fluid ; for tho very tissues which provido for special rise
of turgescence from functional causes are also those which present the greater structural
facilities for transpiratory loss. In yonng tissues we have a relative cxcess of
protoplasm providing for the presence of osmotic products iu the cell-sap ; but at the
same time the facilities for loss of iîuid ia consequence of pressure or evaporation ai-e
greater than they are in older tissues iu which tho foi-med materials—the cell-walls—
are more matured. Hence the fullest solar stimulation will be incapable of giving
rise to the full assumption of tlie diurnal position, unless it be associated with certain
conditions of general supply and loss of water. During the day, the preciso position
assumed by nyctitropic loaves will be determined partly by degree of solar stimulation,
and partly by conditions of root-supply and evaporative loss. At sunset we have
the removal of solar stimulation, but, under ordinary circumstauccs, no immediate
cessation of transpiratoiy ' loss ; and hence the position assumed by the leaves is not
the neutral one determined by persistent protoplasmic activity apart from transpiratoiy
loss; but passes beyond this, so as to reach a nocturnal maximum determined by the
fact that the tissues in which the fall of functional activity is greatest are also those
allowing jnost readily of transpiratoiy loss. Subsequently, as trauspii-atory loss diminishes
and gradually disappears, the tissues will naturally acquire the degree of tm-gcscence
corresponding to the osmotic capacities of tho cell-sap apart from the addition
of assimilatory products imder tho influence of solar stimulation, and the tissues which
make for the diurual position will become relatively stronger than they were when
subject to transpiratory loss. There will necessarily be a general rise in turgescence,
but the rise \vill be greater iu those masses of tissue which are most afiected by
transpiratoiy loss, and hence a reversion towards the diurnal position takes place. The
fully devclop3d diurnal position is not, however, attained until solar stimulation comes
in to induce an excess of osmotic property in tlie cell-sap of the masses of tissue of
greatest functional strength.
Tho actual amount of displacement occurring in individual cases is further affected
by tho ari-angoment of the opposing masses of tissue iu the motor organs. In some
cases, we find iho massos making for the diurnal position so situated that they are
aided by the action of the leverage of other parts of the leaf ; in others, we find that,
in addition to overcoming the action of their opponents, they have to oppose that of
the leverage; and it is clear that, other things being alike, a greater displacement will
be effected by the same rise in turgescence in the first instance than in the second.
In endeavouring to accouiit for the movements occurring in any individual case, therefo)
e, the precise ai-rangement of the opposed masses of tissue in the motor organs
must be considered not merely locally, but in relation to other parts of the leaf.
CHAPTER VII.
I h i s t n i c t m - a l { i t c i i l i a r i t i i s of ike motoi' © i g i i n s of f t i j c t i t v o p ic
plenties.
In consideiing the question of the movements of stomata, it was pointed out that tiie
elements which make for the diurnal position—the guard-cells—differ from the surrounding
epidermal elements in certain featur-es; we have now to endeavour to determine how
liN THE MOTOR OliGANS OF LEAVES. 51
for differences of a liko nature can be recognized in the case of nyctitropic tissues
generally. In the case of the guard-cells, it was easy to show that we were dealing with
elements characterized hy their relative youth, functional strength, and structural weakness
in certain directions; it now remains to ascertain how far similar distinctive features are
present in tho masses of tissue which make for the diurnal position in other examples of
nyetitropism.
the lii
lamii
Fig.
That the masses of tissue whicjr make for the dhn-nal position of nyctitropic leaves
are younger than those making for the noctumal one, is very clearly indicated in most
coses hy tlie fact tliat in tlie as,-mmption of the nocturnal position there is a manifest
tendency to reversion to one which wos a permanent po.sition at a veiy early stage of
development of the leaf, or the diurnal one during on earlier portion of the period during
which movements occur. In the leaves of most species of BaiJimà immediately before
movements begin to occur, Ihe lomium are depressed at angles of various degi-ees from
10 of the petiole and have then two halves fo'dod up, so that their' upp°er suiiaces
1 close contact with one another. When nyctitropic phenomena begin to api>ear, the
as rise and unfold during the day and sink and fold up at sundown (Plate II,
0). In Cassia alala the pinnas just before they become motile, ore folded Kke
Bouhiuia leaves and have tlieir midribs closely applied to the sides of the rachis and
directed upwards and forwards at an acute angle to it. When movements set in they
consist of diurnal separation of the upper surfaces of the two halves of the laminas and
divei-gence and depression of the midribs, and of nocturnal folding of the lammas and
convergence and elevation of the midribs. In Cassia smmtram. the pinna;, immediately
before they begin to show periodic movements, ore disposed as in the previous species,
save that their midribs, in place of being elevated, are deeply depressed beneath the plané
of tho rachis; and here we iiave diurnal unfolding of Ihe laminm accompanied by
elevation ond divergence of the micUbs, and nocturniil folding cohioiding with convergence
and depression. In the leaves of FithecolMim saman, which are ultimately
characterized by the great development ond persistence of their nyctitropic movements,
the permanent position of the various parts at a very early stage of development is as
follows. Tlie lower port of the primary rachis is directed at a very acute angle to the
a.xi8 and tho distal pari is ot first somewhat ciiiwed inwards (aide Plate I, Fig. 13), and
subsequently, as tho weight of the secondary rachises and pfaimles increases, abruptly
curved outwards. The secondary rachises ore closely oppressed to the primary one and
are directed obliquely upwards, and tlie pinnules are fully e.lpanded and ai-e related to
the secondary rachises as the latter arc to the primary one. Somewhat later, as the wood
strenglhens, the primary rachis straightens out, still retaining an acute angle to the axis
Ihe secondary rachises ot the same lime sink and are ultimately depressed beneath the
plane of the primary one, wliilst the pinnules retain their original an-angoinent. When
nyetitropic movements sot in, they are not complicated by any folding ond unfoldin.. of
t le pinnules, as these are from the outset fully expanded, ond the diurnal position is
eharacterized simply by divergence of the primary rachis from the axis, divergence and
elevation of the secondary racliises, and depression of the pinnales, so that their surfaces
come to look upwards and downwards in plate of inwards and outwards. The entire
any secondary racliis here, as iu the case of many other bipinnatc
their movements, clearly corresponchi to
that of a Oassici, whilst in other cases
ig iiicvements corresponding to those of
series of pinnules (
leaves, both in the!
original orrangement and i
the lamina of one pmna in a pinnate leaf such a
we find them each originally folded and exhibit
. Iloi . TI.