THE CATTSiS OF FLITCTUATIOXS IN TmiGESCE>^Cfi
individual pinnre. At suuset we find the primary i-iiclds becomiDg extremely converged
to tho axis, sometimes iudeetl so much so tliat the upper portion actually crosses the line
of the latter, the secondary rachises couverging and sinking and the pinnules once more
nsmg- so as to bring their upper siu-faces into close contact, and their midribs to
n^sun^e a Ime directed upM-ards aud somewhat forwards froar the plane of the secondary
rachis to wiucU they arc attached. Here erery part of the leaf from the primary
vacluses to the pmnnles moves, aud in doing so, dim'nally departs from, and nocturnany
reverts to, what was the permanent position at an earlier stage of evolution.
In the case of Mimosa jmrlica, as in that of IHthccolobmm saman, extensive periodic
movements take place in all parts of the leaves. Immediately before they begin to occur,
the permanent position is as folIo\vs:-The primary petiole is ^^'idely divergent from ih'e
axis, the secondaiy petioles arc closcly converged to one another, and oitlier follow' the
line of tho primary one, or ascend slightly from it, whilst the piiinules arc, as they have
linen from the outset, fully expanded, and have their midribs closely convoA-cd to the line
of the secondary rachises and directed upwards and forwards, so that the upper surfaces
of the laminic of the opi^osite members of the series arc in immediate contact. W]^on
movements are established, they consist of diunial convergence of the primary petiole
towards the line of the axis, diurnal divergence of the secoudai^- rachises from one
another, and diurnal divergence, depression, and a certain amount of rotation of the
pinnules, so that their midribs come to be more or less in the same plane with, and at
right angles to, the secondary rachises, and their superior and inferior surfaces to look
directly upwards and downwards, alternating with noctmnal divergence of the piiinary
petiole from the axis, noctui-nal convergence of the secondary rachises to one another, and
nootunial revei-sion of the pinnules to their embryonic position. The movemeuts in
Mimosa ptcdica and Pithecolohium saman arc thus essentially similar in their nature, in that
in every part of the leaves they involve a diimial departure from, and a nocturnal
reversion to, a previously permanent position. They differ, however, in detail, becausc in
tho one case tlie latter im2>lios divergence and in the other convei-gcnco of the primary
petiole in relation to tho axis. Tliis in no way affects the question of the noctiu-nal
position being one of reversion, but the difference is at the same time important, because
it implies a great difference in tlie stability of the diurnal position in the two cases. Tlie
diunml position is in any case an unstable one, depending as it does on a temporary
victory of functional over passive structm-al resistance; and it is evident that in an
instance like 3Imosa jmdica, in wMch tlie dim'nal position of the primary pulvinus implies
that the mass of tissue maldng for it mnst overcome not merely the resistance offered by
the opposing part of the pulvinus, but also that of the le\'cragc of the distal parts of tho
leaf, the instability must be greater than where levei-age and special rise in turgidity, due
to fanclional causes, co-operate in determining the position. In Pithecolohium the mass of
pulvinar tissue in wMeh fiuictional activity preponderates lies above the flexible fibrovascular
axis of the main pulvinus, whilst in Mimosa piidica it lies below it. The diurnal
excess in tm-gescence in the ono case makes for divergei\ce and in the other for
convergence of the petiole. But the weight of tho distal parts of tho leaves is conslantly
making for divergence, and especially so during the day, owing to the increase in
turgescenco and weight which takes place under the influence of solar stimukiion of
functional activity. In ]?itlieGolohium, tlierefore, the dim-ual relative increase in strength
of the functionally stronger puhinar tissue, and the increased leverage of the distal parts
of the leaf coincide in making for divergence of the petiole, whilst in 3iimosa pudm
l y TI!E MOTOR OBGAXS OF LEAVES, .-53
the pulvinar tissue makes for convergence and the distal parts of the leaf for divngence.
Under tlieae circimistances, it need not surprise us to fiird. conspicuous changes in tho
]josition of the leaves of Mimosa taking place under the influence of factors affecting
pulv-inar turgcsocnce which are incajjable of producing any appreciable results in the case
of the leaves of PWmohhitm.
In some cases, as in tho pinmiles of Pithecolobium saman and Lmccem qlaucu, the
movements during a great part of tlie life of tho leaf effect a complete nocturnal restoration
of what was a pei-manont position at an early stage of evolution, and when they
ccase to do so, determine a simple incomplete restoration; but in other cases, as time goes
on, the development and increased strengtli of certain masses of tissue introduce complications,
and wo no longer find simple complete or incomplete restoration taking place. In
the pinnules of Fitlmolobiim aud Leuoccna glauca we are dealing with laiaiua3 wluch
from tlie outset are fully unfolded, and tlie movements w-hicb they exhibit are simply due
to the subsequent development of a mass of parenchyma over the insertion aud base of
the midiib, which, Avhen its turgescence rises under tlie influence of solar stimulation,
is able to determine the divergence and depression of the lamina, liut in cases, sncli as
the pinnte of Cassia aJata where the lamiua is primaiily folded and sliortly afteiwai-ds
becomes permanently expanded, the characters of the movements become modified when
the latter condition has been estabKshed, factors wluch previously determined expansion or
folding of the lamina now coming to determine movements of rotation in -it. This
modification of movement may be more satisfactorily dealt with in considering tho
particular phenomena presenting themselves in individual leaves, aud is merely alhuhid
to hero in order to indicate that we may not in all cases be able at once to rccogiiize that
tlie noctiu-nal position corresponds to a reversion to an originally perinauont position. In
all cases it corresponds to a tendency to such reversion, but the degree and simplicity
with which the process is earned out is often greatly affected by structural modifications
wliich hare been established subsequently to the period at wlii(Ji the priuiary permanent
position was first departed from.
The fact that the masses of tissue making for the diunial position of the various
parts of nyctitropic leaves, agree witli the corresponding elements deteimimng tlie diurnal
condition of the stomata in being relatively young, is thus indicated by the characters of
tlie movements which they give rise to; but we ai-e' fortunately not obliged to remain
satisfied with evidence of this nature, as in many instances we find most conspicuous
structural indiccs of the same fact. Figures a to g, Plate I, Kg. U, show the appearances
presented by the primary pulvini of seven successive leaves on a shoot of nthccohhium
saman, and in passing frou\ 1, which is the youngest, to 7, which is the most
matiure member of the series, the excessive development of the mass of tissue on the
upper aspect of the inilvinus dm-ing the later stages of evolution comes out most
distinctly. The evidence afforded by sections through the tliickness of pulvini of different
ages is equally convincing, for these show that at first the development of the superior
pulvinar pad lags behind, aud at a later stage overtakes that of the inferior one.
(Plate I, Fig. 15; Plate IV, Figs. 1, 2.) So, in tho case of the tertiary pulvini, the
superior pad is at first very inconspicuous, but ultimately appears as a prominent
mass of vivid green tissue. In cases where, as in the primary pulvini of Cassia alata
tho fibro-TOscular tissue does not assume the foim of a simple unlignified central cord'
but remains arranged in a ring and undergoes nomial processes of lignification,
conspicuous 1 eriodic movements do not manifest themselves, and the continued oTowth