i 3 2 
R. C. Knight. 
must be observed that the theory requires the assumption that the 
water-retaining properties of the outer regions are different from 
those of the more deeply seated tissues of the plant. If the water¬ 
holding capacities of all parts of the plant were similarly affected 
by changes of illumination, then the relation between the water- 
content of the epidermal layer and that of the internal tissues 
should remain the same throughout the day, other conditions being 
equal. This, according to Mrs. Shreve’s theory, is not the case, so 
that it is necessary to assume a difference between the effect of 
light on the epidermal cells and the effect on the internal tissue, in 
order to explain the difference of behaviour in light and darkness. 
In 1913 the present writer, at the suggestion of Professor 
V. H. Blackman, undertook a series of researches on the various 
factors concerned in the control of transpiration. The work has 
been in progress since that date, but as it has now been unavoid¬ 
ably interrupted, it was thought desirable to give some account of 
the methods in use and to summarise the results obtained. 
Attention was first directed to the influence of stomata on 
transpiration, and the methods of measuring the aperture of 
stomata were considered. The porometer method of Darwin and 
Pertz (9) was fixed upon as the most attractive, and a considerable 
time was devoted to the testing and improvement of the manner of 
operation of this apparatus. A convenient form of the porometer 
was devised, and this was described in 1915 (13). In conjunction 
with Mr. Laidlaw this apparatus was modified to render it self- 
recording (16), thus obviating the necessity of continuous attention 
when a continuous record is required. Self-recording porometers 
had already been described by Balls (4) and by Neilson Jones (25), 
but these were comparatively elaborate, whilst that devised by 
Laidlaw and Knight is simple in construction. 1 
With the help of these forms of apparatus the porometer 
method was tested with a view to determining its efficiency. The 
results were reported in 1916 (14), and the following are the 
important points which were brought to light. 
The pressures used to draw the stream of air through the leaf 
should be as small as convenient to avoid the possibility of altering 
1 It may be mentioned here that during the greater part of 1916 a new 
form of the recording apparatus has been in use with very satisfactory 
results. The principle is that of the Mariotte bottle, as in the apparatus 
described in the paper cited above, but in the new form the pen of the 
recorder is actuated by the discharge of the bubble from the end of the air- 
intake tube, not by the fall of the drop from the end of the dropping-tube as 
in the earlier apparatus. By this means the apparatus can be used satisfac¬ 
torily under ordinary conditions without a constant temperature bath or 
a water jacket. 
