19 2 Laboratory Notes. 
The advantages of this model lie in the fact that it demonstrates 
the influence of changes of turgescence in leading to the opening 
and closing of the pore, and that the inner faces of our model guard 
cells consist of thicker material than the outer. On the other hand 
we are of course dealing with cell-complexes in place of cell-units, 
but as the method is intended only to demonstrate to a class the 
mode of action of the stomatal mechanism, this does not seem to 
us to be a vital objection. 
2. Continuity of the aerating system of the plant .—The ordinary 
wash-bottle method 1 of illustrating this important fact depends 
entirely on obtaining a suitable leaf, and has the further drawback 
that it is not readily demonstrated to a class. By utilising an air- 
pump the continuity of air-spaces in the plant is of course readily 
shown, but the method about to be described only involves the use 
of materials that are to be found in any laboratory, and moreover, 
operates for some considerable length of time without attention. 
For the purpose of this experiment we use a round-bottomed flask, 
provided with a rubber cork, through a single hole in which a piece 
of glass tubing twice bent at right angles is inserted (Fig. 1, A). 
The one arm of the glass tube projects only for a short distance 
below the cork, while the other (free) arm is considerably longer. 
To the open end of the latter a branch bearing leaves or a single 
leaf is fitted by rubber tubing so as to make an air-tight connection, 
the cut end of the stem or petiole extending into the tube above 
the level of the surrounding rubber (cf. Fig. 1, A, a). Prior to doing 
this the long arm of the glass tube is partially filled with a column 
of water extending some four to six inches above the cut end of 
the stem or petiole (Fig. 1, A, h). The rubber cork with attached 
tubing is now taken out of the flask, and the latter is then half 
filled with water, which is heated until it has boiled for a minute 
or two. While the flask is still filled with water-vapour the rubber 
stopper is fitted into it, and the flask is placed in a vessel of cold 
water. The reduced pressure within the flask resulting from the 
condensation of the contained water-vapour leads to a strong 
suction, and as a result a stream of air-bubbles commences to arise 
from the cut end of the stem or petiole and to ascend through the 
column of water in the longer arm of the glass tube (cf. Fig. 1, A). 
This goes on for several hours, whilst the flask slowly cools. The 
stream of bubbles is so pronounced that the experiment is readily 
visible to a class of moderate size, whilst the result of the experiment, 
for demonstration to larger numbers, can easily be projected on to 
a screen by the aid of a lantern. 
3. Recovery of a flaccid shoot .—The same apparatus can also 
be used to demonstrate the recovery of a flaccid shoot on being 
injected with water. For this purpose the upper end of the shoot 
is removed and the cut surface thus produced is attached as before 
to the long arm of the tube, whilst the lower end of the shoot is 
placed in a beaker of water. In order to prevent the entry of air 
into the tissues by way of the stomata both surfaces of the flaccid 
leaves should be vaselined. As a consequence the suction generated 
within the flask (as in the previous experiment) operates in causing 
a rapid flow of water through the shoot. After a little time the 
leaves become turgid. 
1 Detmer and Moor. 
“ Practical Plant Physiology ” (1909), p. 172, Fig. 61. 
