4-1 THE CAUSES OF FLUCTUATIOXS IN TUEGESCENCE
simplest form, it is advisable to study it some-what closely ere proceeding to the
cousiclcratioQ of cases in which the movements are no longer determined by tho mutual
relations of individual cells, but by those of complicated masses of tissue. As eveiy
one is aware, the opening and closure of the st,omatic orifices are duo to the fact
that, imder certain conditions of illumination, root-supply, and transpiratory loss of
water, the guard-cells become so turgid that they are able to determine a certain
amount of displacement of the surrounding epidermal elements; and that, in the absence
of these conditions, they lose turgidity to a greater or less degree, and, being no longer
able to withstand the recoil of the neighbouring tissue, they allow it to approximate
their inner siufacos and more or less completely to close the interspace between them.
This, it is evident, implies that under favourable conditions the guard-cells become
more turgescent and, for tho time being, stronger than tho other epidermal elements ;
for did they not do so displacement of the latter would be impossible. "We have then
in the first place to ascertain what are the distinctive features in tho guard-colls as
compared with the other epidermal elements, which are associated with this capacity for
temporary increase in strength. And when we have done so we find them to be the
following :—Isi, they are younger than their neighbours; 2nd, they are licher in
protoplasm; 5rd, they are conspicuous as containing a relative abundance of chlorophyll
and starch; ith, they are, as Kienitz-Gerì off has shown,* distinguished by the
fact that there is no continuity between their protoplasm and that of neighbouring
cells. The three first of these characters are characteristic of the active elements in
nyctitropic tissues generally; but the fourth is naturally absent in cases where movements
are determined, not by alterations in the tm-gescence of isolated cells, but of
masses of tissue. In the case of the guard-cells it is necessary that conditions providing
for the excessive tm-gidity of individual colls should be provided, and accordingly
wo iind the cell-cavities closed and the protoplasts isolated; but in the case of masses
of active tissue there would, of course, be no advantage attained by the isolation of
the individual units entering into their constitution.
That the guard-colls belong to a younger generation than the surrounding common
epidermal cells is a well-recognised fact. In the first edition of the English
version of Sach's "Text-book of Botany" we find it stated that "as the stornata do
not arise until rather lato, that is, during or after the expansion of the intornodes
and leaves, their arrangement is partly dependent on the originally elongated form
of the epidermis colls," and that " tho origin of the stornata is always the result of
the formation of a mothea'-cell, first of all by division of a young epidermis cell,
which is sometimes preceded by several preparatory divisions in it or in adjoining
epidermis cells; and this mother-cell becomes more and more rounded oS, and the
gTiard-cells are formed from it by division." t
It is unnecessaiy to quote any special authority for the fact of the special
abundance of protoplasm, and particularly of chlorophyll and starch, in the guard-cells
as compared with other epidermal elements, as it is one wliich is evident to the most
casual observation.
• Dio Protoplasmarerbiudmigi
Bot. Zeit. 1891.
ilieD bennchbarlen Gewebeselementea dec Pflun
+ A Test-book of Botany; Sacks. Bonnctt and Djcr, 1st ed., pp. 75, 87.
IN THE MOTOR ORGANS OF LEAVES. 45
In the guard-colls we have to deal not merely with younger elements, and
therefore with elements which are for a time structurally weaker than tho surrounding
epidermal cclls, but with elements which in certain areas remain permanently
weak, owing to tho characters of their walls, which, as is well known, remain permanently
thin, extensile, and elastic on their lateral faces. They are not, however, merely
structiu'ally weaker than theii- neighbours, but they are also jjiiysioiogically stronger
on account of their relativo oxccss of protoplasm, and specially of chlorophyll whicli,
under the influence of sunlight, determines an excess of assimilatory activity and with
this an e^icess of turgescence, due to the rise in osmotic capacity wliich the products
of assimilatory activity determine in tho ccll-sap, and which attains a specially high
degree of development in consequencè of the complete closure and isolation of the cell
cavities. An equal rise in txirgescence does not occur in the common epidermal cells,
owing to thcii' poorer protoplasmic content and deficiency in chromatophores; and hcnce,
under the influence of sunlight, the guard-cells become for the time being relatively
stronger than their noighboiirs, and are able to determine a certain amount of displacement
of them. When assimilatory activity is no longer stimulated, the manufacture
of osmotic products falls, and with this the turgescence of the guai-d-cells ceases
to be excessive and the elastic recoil of their own walls and of the surrounding displaced
cells causes the tissue to resume the position proper to its passive condition as
determined by tlie structm-al strength and arrangement of its constituent dements. The
common epidermal cells are structurally stronger but functionally weaker than the
guard-cells, and hence are relatively weaker during periods when the tissue is exposed
to sunlight than they are wlien it is in darkness. Hie guard-cells, on tho other
hand, are structurally weaker but functionally stronger than their noighbom's, and
consequently attain their maximum relative strength during periods of insolation.
The position of the cells during darkness represents tho outcome of the structural
features of the tissue along with the degree of turgescence .persisting apart from
photic stimulation of its protoplasts. The incidence of sunlight determines a greater
rise in turgescence in the guard-cells than in the surrounding elements, and consequently
makes them relatively stronger than the latter are, and in doing so enables
them to causo a certain amount of displacement of these. The removal of solar
stimulation, on the other hand, gives rise to a general loss in turgescence; but, the loss
in the guard-cells being greater than that in the common epidermal cells, the latter
now become the stronger and return to tho position Vihich they occupy apai-t from '
the action of the stimulus. Tho opening» and closui-e of tho stomatic orifices are,
therefore, tho results of the structural and physiological differences existing between
the guard-cells and the other epidermal elements ; the relative structural weakness of
the former determining closiire in the absence of light, and the relative functional
weakness of the common cclls secm-mg that opening shall occur on the incidence
of solar stimulation.
It is clear that these inovoments, which are really nyctitropic when due to the
presence or absence of light, may arise independently of any variation in conditions of
illumination, so long as the variations in turgescence, which are their immediate causes,
are presei^t. As we have already seen, the actual degree of turgescence in any tissue or
tissue clement at any given time depends not merely on the osmotic capacities of the
cell-sap, but on tho amount of water available to satisfy these. Accordingly we find
that, oven under conditions of maximum solar stimulation, tho stomatic orifices are more