Ò4 THE CAUSES OF PLFCIUATIONS I S TUEGESCEKCE
of «10 upper or asHla.y pad during the ktcr stages oS tlio eTolntioi, ot the leaf merely
serves t . a.d tke rnereasmg leverage of distal parts m deter.dm„g a gradual a„d prog.
essive morease m drvergoBoe from the li,>e of the axis. In sueh casos the arrangenLt
and e W t o r of the vascular hmdlos of the pulviuus are such that mere fluctuations in
tnrgicMy are unable to givo rise to corresponding displacements of the petiole, and it is
only as tho result o continued growth that the degree of diyergcnoe is dote^ined.
1 here is thus both dj-uamical and stnictural evidence that the masses of tissue in
ho motor organs which make for tho diurnal position arc younger, or contain a larger
nuniber of young elements, than those which mal.o tor the nocturnal one; and this in
Itself aluiost necessarily iniphcs that, a, a whole, they are sti-uctnrally weaker and
functionally stronger than the latter are. T.ldng any ordinary unit of vegetable tissue,
there can he no doubt that, as a rule, the proportion of aCivo protoplasm in it tend^
to annimsh, and that of formed material to increase, with increased age. Tho formed
iiiatenal may take the shape of secretions of various kinds, or may enter into the con
stitntion ef the celhwall. In any case there is a tendency to di.ninutien in t i e t i a t " :
amount of protop.asm, and, in cases where continued additions aro mado to the coll-wall
hese miply mcrea«d structaral strength, and hence, speaking generally, the larger flm
mibei ot young e ements ni any tissue, the greater will its functional power be; and
he laiger the nnmber of old elements, the greater its stnictm-al power. It follows from
Ins that m H v i n . m which we have to deal with opposing n,asses of tissue differing
ircm one another ni regard to tile period during wldch activo growth continues, we are
necessarily deahng with an oppo.sition of fimcrional and structural strength, the less
malni. tissue homg functionally, and the more mature structurally the sL^ger. St
the e£f.cl produced hy exposure to stimulation must be proportionate to tho a L u n t of
matenal to be stnnulated; and hence, under exposure to any conditions hnplyhig protoplasactauty
t t a n the structura ly stronger one, and where the increased activity leads to
ncie sed turgescence as that induced by solar stimulation nnoquivocally doL, it will
uudeigo a coriespending mcrease in relative strength. Under solar stimulation a general
and enables .t o overcomo the rosnstance in the structurally stronger one, which determme
the position apart from solar stimulation. The corddiko anangemei t of the m l
vascular tissue and the absence of lignitication in typical pidvini, ¿ovide facilities r
the action of the opposed masses of parenchyma on one another, and periodic movements
consecinently arise in connection with fluctuations in tm-gescence c l o i d i n g wrth »
incidence or removal of solar stimulation.»
The masses of tissue which make for the dim:nal position are not merely richer
i i ^ p r o t o p h « m , j m d , therefore, endowed with greater general functional activiy than
•Snchs (Veriest
tursesceBce under tl
actually present, is t
a coDdition of more
of Ibo nocturnal p
connected witli doc
ilie motor apparnt
3pts Miilardefs and Pfeffer' S.776) s statements that tlie pulvini do not lose but p
iufluenc© 0/ darkness, because their rigidity
conclusi-ve evidence of increased turgescence
able passive equilibrium than they possess u
follows defective supply or escessir
}ased urgcsccnci
1 be made
Q the r organ
o dect d tin
he subject o! direct
do ill ayotitropic leavci
elcmejils,
led then. But mere increased rigidity,
as it may merely indicato that the tissues have
idcr the infiueacc of ligln, aud tliu face that an
• loss 0! Tratet seems conclusively to show
In the caso of the stomi.ta in wl.ich tho c
.bservation, and in whieh the movements c
wo do not find darlcness civiu" riso to in,
if it be
passed into
IN THIC MOTOR ORGANS OP LEAVES. 55
fchcii- opponents, but arc also as a rule specially riclier in cliloroi)hyll, and tlicreforc
specially subject to the influonco" of solar stimulation as a source of increased turgesuenco
dependent on the manufactiu-e of assimilatory products. Tins in many cases, as in those
of the pinnal pulviui of Cassia ahta, and in the secondary aud tertiary pulvini of Pilhecololiim
saman, is evident to the most c\u-sory inspection o-^-ing to superficial differences
in coloiu-, and iu all it can be readily ascertained by means of imcroscoi>ic sections,
especially if these iiavo been momentaiily immersed in a 2 per cent, solution of osmic
acid so as to iix the colouring matter in the cLromatophores. (Plate J, Pigure 19;
Plate II, Figs. 3, o—S; Plate IV, Figs. 3, 5—9).
The diurjially donjinant tissues in the pulvini of nyctitropic leaves are then
distinguished from the nootumally dominant ones by precisely the same features
which distingiush tho guaa'd-cells of the stomatic orifices from the common epidermal
elements.
Tliey are distinguislied by their youth, or by the large proportion of relatively
young elements which they contain, by theii- relative structural weakness, and by their
relative functional strength, especially in relation to solar stimulation; and tiie movements
wHch they givo rise to are of essentially similar origin to those caused by
tlie guard-cells. Just as the guard-cells are able to effect a displacement of the
surroimdiaig tissue when exposed to sunlight because of their tempora.y excess in
turgescence, so aro the masses of tissue in the puh-ini wliich make for the diurnid
position able to effect displacement of their opponents under similar cii-cumstances.
I h c condition of the stomatic orifices is regulated not merely by fluctuations in tho
degree of solar stin.ulalion to T^'llich the tissues are e;<posed, but also by the coincident
conditions of root-supply and trauspiiatory l,.ss of water; and the snme holds good in
regard to the position of nyctitropic leaves. Where trajispii-atory loss is in excess,
and yet falls short of the degree Jiecessaiy to causae general wilting of the tissues,'
we find both stomatic orifices and nyctit:-opio leaves assuming the nocturnal positionand
m both cases a resumption of the diurnal one follows a restoration of llie
norii.ah relation between general loss and supply of fluid, whether this be attained
by means of diminished transpiration or increased root-supply, The excess of
functional acti^•ity in the guard-cells, and the masses of pulviuar tissue making for the
diurnal portion, provides that they shall undergo a greater increase iu tiu--escence
under solar stimulation th.n their opponents do, and tliis implies a temporary increase
in the resistance which they present to them; but at the same time the excess of
stractural strength in the common epidermal elements and the pulvinar tissues making
loi tiie noetural position giyes them a relative protecf ion fi-oin the action of conditions
ui.plymg an escoas of tran.piratory loss over root-supply of water. The result of
H™ natm-ally is that, where this exeess rises high enongli, a resumption of the
noetm„al position oeeu™ in spite of the eontinued stiinnlation of the more aeti™ tissaes,
becaase these are no longer able fnlly to satisfy their osmotio eapacities. Where
c^iditions of general supply and loss of tlaid hold a normal relation to one a n o t h e r ,h
diiiiml position ,s seeured by the greater osmotio oapaeity of the more aeti™ tisanes