Porometer in StomcUal Investigation. 65 
the same time the control reading has changed from 11-7 to 9*8, representing 
an increase of 16 per cent, in the rate ; the effect of vaselining is clearly much 
greater than the readings of A indicate. 
A modification of this method has also been employed with similar 
results. In this case the whole leaf outside the chamber was vaselined, 
except a long rectangular area, the shorter side of which was immediately 
adjacent to the leaf-chamber. Readings having been taken, the length 
of intercellular spaces to be traversed was increased by vaselining a portion 
of the rectangle nearest the chamber. In this case an increase of 3*0 cm. 
produced a decrease in rate of 35 per cent., the control remaining practically 
constant throughout. The plant used was Eucharis amazonica , which, 
as well as the other species of Eucharis used in the experiment previously 
described, has stomata only on the under surface of the leaves, with occa- 
sional ones on the petiole. The case of hypostomatous plants of this type 
is somewhat different from that of plants with stomata upon both leaf- 
surfaces, such as species of Helianthus , Saxifraga , and Richardia which 
have been used in later experi- 
ments. In the latter types the 
air-stream is at liberty to enter the 
leaf from one side and pass straight 
through into the chamber on the 
other side, reducing the length of 
intercellular spaces traversed to 
the thickness of the leaf. A vase- 
lining experiment, as described 
above, upon an amphistomatous 
leaf would require the whole of the 
stomata upon one surface to be blocked before measurements could be made. 
The question of amphistomatous leaves will be further discussed later. 
2. A more convenient and efficient method of carrying out what 
is practically the same experiment as that described above has since been 
used. It depends on the use of a special type of double chamber of the form 
shown in Fig. 4. This consists of one chamber within another, so arranged 
that both can be fixed to the same leaf, each being provided with an 
independent outlet. Those used in the following experiments have been 
generally constructed in two parts and joined by means of rubber tubing or 
sealing-wax (at A in Fig.). The double chamber is fixed to a leaf and 
readings are taken from the inner chamber in the usual fashion. The 
air enters the outer chamber at C, passes into the leaf at D, and is drawn into 
the inner chamber at H. This constitutes an ordinary porometer reading 
from a single chamber. The outlet C of the outer chamber is now closed, 
and air must now enter the leaf at E — beyond the outer chamber, and 
to reach the inner chamber must traverse a longer path within the leaf, 
Fig. 4. Diagram showing the form of double 
chamber used. 
