3 2 THE CAUSKS OP PUTOIUATIOXS IN IUKGESCESCB
transptafory loss in weight and lcYor»go of the pinn», »nd thi. is of com-» dmvalent
to increased relative strength in the portions of the primaiy pulvinus winch make tor
eloTation.
The excessive elevation of the primai-y petioles during the later lionrs of the night
is Owino- to a similar d i s t m t a c e in the relations existing between pulvinar strength and
distal leverage. The diminntion in distal leverage in this case is not, however, owing
to excessive transiraatory loss in the cKstal parts of the leaves, but to continned absence
assimilation and coincident continned respiratory loss. 'I'he position of the pnmary
petioles at simsot is no donbt partially dotennined by the fact that the infei-ior mass of
p a r e n c h ™ contains an excess of tissue Kable to rapid filtrative loss of flind on the
cessation of the active assimilatory formation of unstable osmotic products consequent on
removal of solar stimulation, wldlst transpiratory loss continues to proceed, and a oortam
amount of depaitoe from the maximal nocturnal position might natm-ally be expected to
oecnr muler tho influence of cessation of transpiratory loss and continued root-supply, the
denaiture would not, however, proceed so far as it docs and lead to maximal elevation just
before sunrise were it not for the steady decrease in weight of the distal portions ot tho
loaf dependent on continned respiratory loss; 'for the hours of darkness correspond to a
period of fastin.. durin.^ which continuous expendilm-e tokos place without corresponding
addition ef new°materiols derived from extraneous som-ces, and oonsoqnently loss in weight
must occur. On renewed solar stimulation the pulvinar tissue increases m tiiigescenee and
absolute strength, and tho inferior mass of parenchyma undergoes relative iiicimse m
strength in respect to the superior one, but not in proportion to the mcreased weight 0
the distal parts of the leaf which takes place in consequence not merely of their increased
„ir.escence, but to the gi-adual aeeumulation of fixed products of assimJation which takes
„lace within them under the infliience of sunlight. Maximal elevation can thus no
lono-er be maintained, but progressive descent sets iu and attains a maximum when he
reiMval of solar slimulation occasions general loss in Im-gesccnce at a period when the
accumulation of the fixed products of active assimilation has attained its maximum. The
accumulation of assimilato.y produeis in the pulvinar ti.ssiies does not render them more
powerful to resist leverag.. save in so far as it afiects tho esmoiic properties of the
cell sap. but tho accumulation w-ithin tho distal parts of the leaf renders them heavier,
and tlwefore more resistant to the elevating action of the inferior mass of pnlvmar
parcuchjina.
No such anomalously excessive departure from the primary noetm-nal position occurs
in the secondary raehises and phmiiles. Tho reversion towards tho noetm-nal position
exldbited by them during the later parts of the night is bmited m extent and is of
m-eci,elY the same nature and origin as that oeoui-ring in nyelitrepm strnctures generally.
I t is'owin-- simply to t h e ' f a c t that, on the removal of seta- stimulation, greater filtrative
less of fluid takes place fi-om those tissues which make for tho diui-ual position than
from their- structurally stronger opponents during the period in which active tl-anspn-a ion
continuos, and that during the suceeeding period of continued absence of .ran.pMtion
and uninterrupted root-supply all the tissues alike have the Opportunity of M l y satisfying
.he osmotic capacity wldch they possess apart from solar stimu atlon ^ '
eertain amount of increase in relative strength in tho tissues which make for the diurnal
l o ' S o n T d wldch had previously sugered most from filtrative less. But in the ease
0 ^ r p i l y pulviiii, this occurs during a period in which the tissue which gains in
I S THE MOTOI! OIIGA>-S OP LEAVES.
i-elativo strength is opjiosed by progressively diminishing distal leverage which is equivalent
to its undergoing progressive and additional inei-enients in relative strength.
The sti-uotm-al features in tho motor organs of Mimosii pttdim wliioh determine the
occurrence of the various periodic movements are par-allol to those which we have already
found to be characteristic of those of other nyotitropic loaves. As in the ease of the
latter tho opposing masses of pulvinai- parenchyma appear at a relatively late peno.l
in the development ot tho leaf and are not developed simultaneously, the development e£
the masses wliich make for the diurnal position lagging behind that of their opponents.
In the case of the jn-imary pulvinus the antecedent development of the mass of tissue
which makes for the nocturnal position is clcai-ly indicated by the fact that, for a
considerable time ere periodic movements set in, a process of contiiuionsly increasing
divergence of the primary petiole from the line of the axis manifests itself, which
ultimately leads to its assuming a permanent position corresponding to the primary
noctiii-nal position of a later stage of dovelopmcnl. At first .sight it might appear as
though the permanence of the position were anomalous, and as though movements ef
nocturnal convergenee similar te those ocouri-ing iu young loaves of PUI„Mim, ought
to attend the noctunial loss in turgeseence of the growing mass of pai-onchyma ovetlymg
tlie vascular axis of the pulvinus. There is, however, no appreciable evidence ef their
oecuiTonce, and as a matter of fact there is no reason why they should occur.
The conditions aro not pai-aUel in the two cases. In Fiihccohhium mmm tlie mass
of pulvinar tissue which loads to divergence has not only to overcomo the resistance
of a relatively strong vascular axis, but also that of an antecedently dovelopod mass of
pulvinar parenchyma malting for elevation; whereas in Mmosa pildica, during the period
at wKoh progressive divergence of the primary petiole advances continuously, the
accumulation of pulvinar tissue mating for convergence has not yet begun to make
its appearance; and the axial ooi-d of vascular tissue is very weak, and fi-oin its
flattened form, specially flexible in a vertical direction, so that, so' long as it is
unaided, it is incapable of producing any appreciable effect in resisting the pressure
ef the axillary parenchyma aided by the rapidly increasing leverage of tho distal
portions of the leaf bearing on the oontimiously elongating petiolai- lever.
In the matm-e primary pulvini the axis consists of a soKd cord of soft flexible
wood, sm-roimded by a sheath of hast and containing no medullary tis.siie. It is
compressed vei-tioally, so as to present an oval figui-e with the long diameter horizontal,
and is sm-i-ounded by a thick mass of parenchyma (Plate V[, Figs. 3, oi). As tile outline
of the pulvinus as a whole is more cii-cular, it necessarily follows that the nias.ses of
parenchyma lying to either side of the fibi-o-vascular axis are thinner than those
wliich lie above and below it. The latter, moreover, are not of equal thicknes,s, for
the superior one is only slightly tliicker than the fihro-vaseular bundle, wiiile the
inferior one is considei-ahly so [Plate V l , Figs. 1, 2, 55). Tho vertical measurements of
the ma.sses above and below the fibre-vascular bundle of the latter, and of the pulvinus
as a whole, in tlie specimen illustrated in the figm-e were as follow
Vortical thiotinosa of entire pulvinus ... ... 1-74 m.m.
„ „ upper parenchyma ... ... 0-57 „
fihro-vasoular bundle ... ... t)-ol
„ „ lower poreachyma ... ... 0-66 „