r i l E OAVSHS OP rLaCIUAHONS IN inEGESCES-CB
c r t am common conditions, sncl, as e x p o s „ e to „ , deprivation oi l i ™^
u „ , , d e f o c v o ro„,.»npply „, ™ , e r , and „ e o h a n i c l a g i t a t i o n , bu, Zlt
manence of l i e oom.non cond.tion, cannot be taken as a n y satisfacto.-y e r i d c L t h«
moto, „,gans differ f r om one another in kind. The phenomena afford no sufficient nroo
of the e . „ t e n c e of any essential difference in the functional properties of the t i s l aTd
he structural featnres of the nrotor organs are snch as sal^ista'ctorily to a c c o n " 'r fc
X Ln tl e T t e r T T ' t " ¿»«»»^ a! r e s d t i ng
e s t ^ i ^ t i ^ a t:
»man one, ba, th.s does not show that the caase of escape is d i f f e r e n t in t h e two caÎes
r h e strnctaral teatares of the motor organs of the leaves of Jfmosa pu,licu afford no
condacted by the motor organs of other nyctitrop.c leaves. They certamly give no
safficrent grounds for assuming that the m o y e m . n t s are i n them, a n y more t h a n T o t h er
ayctrtroprc leaves, determined by alterations i n targescence depending on active c o n t r a"
. . o n of the protoplast, of the tissues, in place of alterations in the osmotic properti s
of the cell-sap, allerat.ons m the relations e a t i n g between general supply and loss If
water, or ahemtions in conditions affecting local filtration; but they do afford a
s a h s f a c t o r y e . p i a n a t r o n of t h e fact t h a t the movements should be more rabidly conducted
t h a n they ate m ordmary nyctitropic leaves. As, however, the belief in the essential
dependence of the movements on the exercise of active contraction by certain of the
protoplasts m the motor organs is so firmly and widely established, it i , desirable that
an attenrpt should be made t o - d e t e r m i n e how f a r t h e pheaomena of movement themselves
real y afford any support to it, and how far they are in any way inexplicable as the
o t r e r l e ° a v e r ° ° " """ "i ^
I n approaching t h i s question it must in the first place be borne in mind that u d -
normal cn-eamstances, the tissues of contain an exceptionally large balk oÎ
hqmd under v e r y hrgh tens.on, and t h a t this tension is maintained in spite of exceptional
facrhtres for general transpiratory loss and for local redistributions of masses o i C a t er
w i t hm certam particular areas. It is evident, thas, that all the conditions of unstable
equilibrium are present in v e . y high degree, and that the general liquid tension must be
l i ab e t o undergo constant fluctuations in degree eo.Tosponding to any fluctuations in th
relations existing between root-supply and transpiratory loss of water. Haberlandt afS.,, !
t h a t the liquid, which under most circumstances escape, in such large bulk f r om the
IN I'HE MOrOK ORGANS OF LEAVES. 9"
tissues on section, is e n t i r e l y derived f r om the system of seive-tubes which he regards a,
constituting a specific, mechanical nervoas system for the propagation of stimulant
impulses inducing active contraction of the protoplasts of t h e motor o r g a n s ; bat, even if it
be so, this doe, not affect the question of the i n s t a b i l i t y of eqailibriam between general
s u p p l y and loss of water. The maintenance of high liquid tension within the s e i vc
t u b e , demands a conslant supply of water to .satisfy the excessive osmotic capacities
of the cell-sap which- they contain. Their excessive targescence only implies
a n additional drain upon the general stock of water which is also subjeet to
excessive transpiratory loss. The presence of any extensive s y s t em of tissue element,
of exceptionally high osmotic captcities, snch as the seive tubes in the bast of Mimom
pwltca, or the laticiferon, tubes of Enplmhm imti,[,mm {Plate VII, Fig. 9), can only
serve to render harder the straggle which other less highly endowed elements have
t o m a i n t am with t r a n s p i i a t o r y loss iu the effort to retain their targescence. The
s t r e am of water along the course of the water-conducting tissues of ilimom pulku,
must m any case bo a n extremely active one if turgescence i , to be fully maintained
under conditions f a v o a r i u g active t r a n s p i r a t o r y loss, no matter whether the diain upon
i t be carried on by all t h e active tissaes in like amount or by a certain class of t h em
i n e x c c s i v e amount. In either case the facilities for t r a n s p i r a t o r y loss remain excessive,
a n d the demand for water to satisfy the osmotic enpaeilies of lire tissues is very high,
so that turgcscence can only be f u l l y maintained under external condition, f a v o n r i n » the
occurrence of transpiration by means of a very active current along the course of
t h e watef-eondactiog system. Whenever atmospheric condition, are ,uch as to favour
transpiration, turgcscence can, therefore, be f u l l y m a i n t a i i c d at a high level only
b y moans of proportionately active root-supply of w a t e r ; and any sudden increase
in t r a n s p i r a t o r y loss or diminution iu root-supply must necessarily disturb the
pre-existing equilibrium and give rise to a loss in turgescence which will, of
course, manifest itself most rapidly and r e a d i l y in those tissaes which present the
greatest texlural facilities for the cscape of water. Loss in taigescence originating
i n any distarbance between the relations of general supply and loss of water will
not occur with equal r a p i d i t y everywhere ; for t h e r a t e of loss must be affected on the
one hand by the osmotic capaeitics of the various tissae. element,, and on the other
b y the stiuctural facilities which they provide for the escape of liquid; and where no
great difference in osmotic properties is present, those tissue, which present the greater
, t r u e t u r a l fucilitie, for t h e escape of liquid from the interior of their t u . g c c e n t elements
wdl lose tnrgeseencc most rapidlj-, and with this will become relatively weaker than
t h e y previously were. But in the motor organs of Mimom pudit«, and especially in
those of them which are most subject to rapid movements, we have to deal with
opposing masses of tissue wliich differ very greatly from one another in the textural
facilities which they provide for rapid filtrative escape of liquid. Any sudden increase
i n transpiratory loss or decrease in the activily of root-supply must necesmrily give
n , o to decreased liquid pressure throughout the water-conducting system generally
and this in its turn to an increased tendency to filtration from the interior of turgid
elements. But the rate of filtralicn in paiticular elements must be affected by the
structural features which they present, and with this corresponding alterations in their
r e l a t i v e strength must be established. Where filtntive facilities are present in very
d i f f e r e n t degrees in opposing masses of tissue, as is the case in the primai-y and t e r t i a ry
. Rcï. BOT. G.ÎED. CAI,CCTT.\ VOL. VI.