THE CAUSES OP FLITCTUATIONS IN TURGESCENCE
must be takiug place in connection with the varying degree in which the difierent factors
are associated -vvitli one another. Every rise in protoplasmic stimulation, so far as it
affects the manufacture of osmotic products; every rise in root-supply; and every fall in
transpiratory loss, must make for increased turgescence: whilst every fall in protoplasmic
stimulation or root-supply, and every rise in transpiratory loss, must tell in the opposite
direction. In the vast majority of cases, however, no readily appreciable results ensue;
and it is only where the fluctuations are considerable, and where the tissues present
certain peculiarities of structui-e and arrangement, that we have conspicuous indices to
their occurrence under normal cii-cumstances.
CHAPTER V.
^hc ifferts prcbuciîi bij fluctuations in iurgcsance.
Fluctuations iu the turgescence of masses of vegetable tissue may manifest themselves
in the form of more or less conspicuous changes in bulk, consistence, colour, or
position. Changes of the first kind present themselves veiy conspicuously in cases of
artificial plasmolysis in linear series of cells. In these a general diminution in bulk
is all that at first presents itself to observation; and it is not until this has advanced to
a cei-tain point, until the elastic recoil of the cell walls has been satisfied; or, in other
words, until turgescence of the cells has ceased to exist, that detachment of the
protoplasts occurs, Changes in consistence associated with fluctuations in turgescence
cannot be better illustrated than by the temporary drooping and wilting which affects so
many leaves during the course of hot, dry days, and which is recovered from when
nocturnal conditions put a stop to transpiratory loss. Conspicuous changes of colour
arc comparatively rare and exceptional phenomena, but some veiy striking instances
of their occurrence may be found. Among these is that which presents itself in
connection with Selaginella seiyens.
Here the fronds, which under normal conditions are of a brilliant green dui-ing
the day, become towards, and for some time after, sunset of a chalky whitish colour. This
t i n t persists for some time and then gradually fades off, so that by midnight it has
been apparently completely replaced by the diurnal one. The cause of these changes
in colour can be readily determined by means of immersing portions of the fronds, whilst
in the various states of coloration, in 2 per cent, solutions of osmic acid for a minute or
two, and then making microscopical preparations from them. This treatment, as Ludwig
Klein was the first to point out, not only fixes the protoplasts of the tissue in the form
which they had when the specimen was taken, but also acts on the chromatophores in
such a fashion as to cause them to retain their normal chlorophyll content and colour,
even when the specimen is subsequently exposed to the action of alcohol and mounted
in balsam, the result being that pei-manent preparations corresponding to the various
changes in colour of the fronds can be obtained.
Before going on to describe the phenomena presented by any such series of
preparations, it may be well to furaish cei-tain details regarding the structure of the
fronds. The greater part of their substance is composed of two stmta of epidermis.
TUE MOTOR ORGANS OF LEAVES. 41
differing, however, from common epidermal tissues, inasmuch as all the constituent cells,
with the exception of those of the maiginal row bounding the frond and connected
witii both stiuta, are abundantly provided with chlorophyll, the cells of the superior
stratum especially containing a variable number of relatively large chromatophores
(plate I. fig. 6]. The superior epidennis over the greater pai't of the frond consists
of short polygonal cells, with wavy margins, and with numerous narrow pits or actual
slits on the external or upper face of their walls (plate I. fig. 3). Towards the
margins of the frond, the cells become somewhat more elongated, and the outer ones
are in dii-ect contact with the common marginal row, to which the external part of
the inferior epidermis is also attached. The marginal cells are, as before said, devoid
of chlorophyll and are of a narrow, elongated figure, some being prolonged anteriorly
into projecting points which give the frond a seiTated contour (plate I. fig. 1).
Their walls are greatly thickened and highly cuticularised, especially externally, and
the projecting points arc almost completely filled up so as to form solid spines.
The inferior epidermis, except immediately over the midiib, is composed of
elongated cells with strong walls and containing only a compai-atively small quantity
of chlorophyll (plate I. figs. I, 2). A few stornata are present along the middle line
of the superior epidermis, and a good many occur generally diffused over the inferior
one. The inferior epidennis and the highly thickened rim form a sort of rigid
frame on which tho superior epidei-mis is highly stretched in such a fashion that,
even were its cell walls extensile and elastic, any considerable alteration in the
capacity of the cell cavities is rendered impossible under ordinaiy cii-cumstances.
Between the two epidermal strata there is a thin layer of veiy open tissue, wMch
is somewhat thickened along the middle line around the single vascular bundle but
which thins out and disappears aro\\nd the edges and distal extremities of the leaflets.
Each leaflet tltus consists of a comparatively rigid inferior stratum, a very resistent
margin, a superior stratum rich in chlorophyll and consisting of cells tho walls of
which are pitted towards the free surface, aud a limited amount of loose intermediate
tissue including a vascular bundle.
The processes which determine the conspicuous changes in the colour of the fronds
take place in connection with the cells of the superior epidermis. When we examine' a
series of preparations of this derived from fronds in maximal conditions of greenness
and whiteness, it becomes at once evident that the protoplasmic constituents of the cells
are in very different states in the two cases. In the bright green diurnal condition
(plate I. figs. G), the protoplasts entirely fill the cell-cavities and the separate
chromatophores which they contain are more or less distinctly recoguisable ; whilst in
the maximal white state (plate I. figs. 5, 7, 8), the protoplasm is shrunken and contracted
away from the cell walls, and the chromatophores are so closely packed together
as to form a spherical mass in which the individual elements are irrocognisable, and
which in many eases is the only visible representative of tho protoplasmic content of
the cell. In other eases a narrow, irregular fringe of colourless, granular protoplasm,
containing a nucleus, may be detected around more or Jess of the periphery of thè
chromatophoric sphere. Between the outer margin of the protoplasm and the cell-wall a
wide spaco intervenes, which in the natural state of the tissue appears to bo occupied
by ail", and it is to the presence of this intracellular aii- that the peculiar silvery or almost
chalky whiteness of the fronds is due. If, finally, wo examine fronds in the green state
\vhich they assume during the latter part of the night (plate I. fig. 9), we find the
Anx. Eo\-. Boi-. Gaud, C a l c u i t a Vol. T I.