63 THE CAUSES OE FLUCTFATIOiNS I S TUEGESCEi'CE
latter become absolutely weaker in the absence of insolation, tliey at the same time
become lolatiToly stronger than they were before, and the previously existing equilibrium
being proportionately disturbed, movements necessarily occur until a new position of
equilibrium has been established.
As we have just seen, the nyctitropic movements of the pinnas of Cassia alata are
very extensive and highly complicated at a certam period in the life of the leaves.
They are, however, entirely absent in very young leaves, and gradually diminish and
disappear in mature ones long before these begin to show any other signs of loss of
vigour. On their onset they mahc thek appearance first in the basal pinnaj and gradually
extend thence outwards along the course oi the petiole. In leaves at the period
when movements are just beginning to appear, all the pinn.-e save the basal ones have
t h e two halves of their lamina! permanently folded up, with their rqiper surfaces in close
contact with one another, and the midribs directed upwai-ds and forwards from the plane
of the petiole. The distal pairs of pinnaj are in dose contact with one another and with
the sides of the petiole, whilst the proximal ones arc more or less divergent, the divergence
increasing in passing downwards towards the base of the racliis. In the youngest
pinnas there is as yet no sign of any accumulation of pulvinar tissue. The midrib lies
in a groove on the upper surface of the lamina, and the basal ribs of the outer side of
the latter emerge clean and clear from it. In the basal paii-s of pinnas the lamina! are
partially unfolded during the day, the sm-faces of the outer half being still almost vertical,
but those of the mner one lying neariy horizontal. Their midribs are still directed
forwards and upwards from the plane of the petiole, but much less so than those of the
distal pinna!. There is already a conspicuous mass of ochreous pulvinar tissue on the
outer side of the keel oE the midrib, and extending thence over the bases of the lower
ribs on the under surface of the outer half of tho lamina. As yet there is very little
accumulation of green pulvinar tissao on the inner side of the keel, but a mass of it has
begun to appear over the base of the upper surface of the lamina! and to extend outwards
along the groove of the midrib. The diurnal position at this period is due to the following
agencies:—The divergence is caused by the presence of a certain amount of green
axfflary pulvinar tissue, tho partial unfolding to the commencing accunnJation of pulvinar
tissue over the base of tho upper surface of the lamina and along the groove of the midrib,
and the persistent olevatiim of the outer half of the lamina to the excessive development
of pidvinar tissue over the bases of the lower ribs on its under surface. As time
advances,' diumal divergence goes on steadily increasing with the increased development
of axillary pulvinar tissue ; expansion of the lamina becomes complete as the
mass of tissue on the upper surface of tho base of the outer half of the lamina
increases in proportion to the earlier developed mass on the under surface, and
permanent as the tissue over the upper surface of the midrib increases in amormt
and structural strength. For a long tune, however, such structural and functional
ditfereuces are present between the earlier and later developed masses of pulvinar
tissue, makiirg respectively for the nocturnal and diurnal positions as to give rise to
nocturnal and diurnal fluctuatioos in then- relative strengths of sufficient magnitude to
induce extensive movemeuts. After a time the functional strength of the axillary
superior pulvinar tissue gradually decreases, whilst its structural strength simultaneously
increases, so that the differences originally existing between it and its opponent progressively
diminish and ultimately disappear, and after this, although general tnrgescence
r IÎÎ THE MOTOR ORGANS OF LEAVES.
continues to undergo diumal and nocturnal fluctuations so long as the leaf retains its
vitality unimpaired, these are no longer accompaniiid by displacements of the pinnoe,
bocause ilicy now affect tlio entire pulvinus alike in place o£ occurring in cxcess in
particular portions of it.
The movements of the pinnas in Cassia alata, although so extensive and complicated
when at their maximum, are always caiTied out comparatively slo\%ly. Owing to this we
fmd. that tlic leaves have hardly begun to show appreciable displacement of their pinnoe
at a time in the evening when those of Mimosa pudica, Leiiooena glauca and Pitkecolohium
samm have already completely assumed the nocturnal posiiion. Correspondingly they
hardly show any appreciable movement under disturbances by wind or rain whicli suflice
to cause complete movements in tho leaves of these plants; and, on being detached
from the axis, in place of immediately showing the effects of arrested water-supply as
the leaves of Minma pudiea and Lcucoena glauca do, they only slowly and gradually
assume tlie nocturnal position. This retardation of movement is tho consequence of the
sti-uctural features of the pulvinar tissues. The masses of pulvinar parcnchyma arc
dense, the intercellular spaces are veiy small, and the cell-walls are provided only with
few and small pits, and consequently any filtrative escape of fluid from tho turgid cells,
whether it be of a nonnal character or the result of loss of osmotic capacity in the
cell sap, or of rise in external pressure, can take place only gradually and slowly. In
order, therefore, to give rise to any appreciable movements, the causes of increased
filtration or of obstmcted water-sujjply must continue to act for a much longer time
than it is necessary that they should act in cases where filtrative facilities are present
in high degree.
Tlie pinnaî of Cama smmtrana also exhibit nyctitropic movements which, although
neither so complicated nor so persistent as those of Cania data, ai-e very conspicuous
durnig a certain period in the life of the leaves. Fully developed leaves have from
nine to twelve pairs of pinnte, which differ from those of Cusda alata in certain
important respects. In place of being practically sessile, they are provided with distinct
secondary petioles measuring 0-12" x 0-04", and they never possess conspicuous laminar
expansions of pulvinar tissue (Plate III, Fig. 4). Nyctitropic movements are very
extensive iu young leaves after they have attained a certain degree of development;
but they very soon diminish in amount, altering in character as they do so, and are
practically absent during the greater part of the life of the leaves. Owing to the very
transitory character of the movements, the nocturnal positions of the successive pairs of
pinna; in one and the same active leaf are by no means so uniform as they are
in the case of Cauia alata, the distal, latest-developed pinnoe often exhibiting the
maximum of movement at a time when the movement in tho basal ones has already
undergone great diminution. As usual, nyctitropic movements are entirely absent up to
a cei-tain period; after this they begin to manifest themselves with rapidly increasing
intensity, very soon attain a maximmn, and then rapidly decrease and disappear.
In pinna; in the earlier stages of development the halves of the laminfe are
permanently folded up and in close contact with one another, and the midribs are
slightly divergent and somewhat elevated from the plane of the common petiole, their
keels at the same time facing divcctly outwards, so that the edges of the folded lamina;
of opposite pinna; are in close relation to one another (Plate I I I , Figure 1). At' this period
there is no evidence of any accumulation of pulvinar tissue in tho secondary petioles.