7o 
Knight. — On the Use of the 
Expt. 52. 10. xii. ’14. 
The plant used was Eucharis Mastersi. Five chambers (a, b, C, d, e) 
were fixed to a leaf and the leaf vaselined as described above. The 
chambers were about 2*2 cm. apart. Pressure difference in the aspirator 
was 18-5 cm. Manometers were read every thirty minutes ; the maximum 
heights reached, excluding capillary effects, were : 
B. C. D. E. F. 
io*o cm. 8*7 cm. 7*3 cm. 6*3 cm. (unvaselined area) 
Rough approximations from these figures give the pressure inside the 
leaf at A as 11-3 cm. below an atmosphere, and at F 5*5 cm. below, i. e. 
the difference of pressure on the two sides of the barrier of the stomata was 
7-2 cm. (18*5-11-3) at A, and 5*5 cm. at F, the lack of agreement probably 
being due to the greater area of leaf exposed at F. 
Thus, whilst in passing from the leaf through the stomata into the 
chamber the pressure change is 7 cm., in passing through a centimetre 
of leaf-tissue the pressure change is at least 0-5 cm., showing that the inter- 
cellular spaces may offer a relatively large resistance to the air-current. 
4. An extension of the pressure-gradient experiment has also been used 
to supplement the methods already described. A series of chambers is 
attached to a leaf, and the remaining leaf-surface is smeared with vaseline. 
The end chamber of the series is connected with the aspirator, and the 
others are provided with stopcocks so that air may be allowed to enter the 
leaf through any particular chamber at will. Whichever path the air-stream 
is made to traverse, it encounters two sets of stomata, one at the entrance 
and one at the exit, but the length of intercellular spaces to be traversed 
may be varied at will by means of the stopcocks, and the difference in speed 
caused by different lengths can be measured in the usual manner. 
It must be noted that for the correct working of this experiment 
the areas exposed under the leaf-chambers must be equal, and to ensure this 
is far from easy in practice. If, however, the areas are approximately equal, 
and if the areas beneath chambers farther from the exit chamber are not less 
than those beneath chambers nearer to it, the experiment will have a quali- 
tative significance. 
Expt. 53. 11. xii. ’14. 
Five chambers, A, B, C, D , E, fixed to a leaf of Eucharis Mastersi , and 
the remainder of the leaf-surface vaselined. 
The areas of leaf exposed under the chambers were in the proportion : 
A. B. C. D. E. 
10 25 25 20 IO 
and the distances between the chambers are indicated below : 
A. 2*3 cm. B. 2-2 cm. C. 2-3 cm. D. 1-9 cms. E. 
