224 Knight . — The Interrelations of Stomatal Aperture , 
transpiration experiments has already been pointed out, and throughout 
the present work the ‘ air-flue ’ elsewhere described (Blackman and Knight, 
4) has been employed to produce air-currents of constant velocity. The 
plants and atmometers under observation were placed close together in the 
glass section of the flue. At first, weighings were made on a balance 
standing on the roof of the flue above the plants, the latter being suspended 
from the balance beam by a wire hook. The fan drawing air through the 
flue was stopped for a period sufficient to carry out the weighings. It was 
found more convenient, when more than two plants or atmometers were 
under observation, to remove each from the flue in order to weigh it, rather 
than stop the fan for so long a period. When porometer cups were 
attached to plants to be weighed, it was necessary, of course, to interrupt 
the record and to disconnect the recording apparatus during weighing. 
Various plants were used for the experiments, including Fuchsia (garden 
hybrid), Antirrhinum, Helianthus annuus,H. tuberosus, Catalpa Kaempferi , 
Ricinus communis , Pelargonium zonale (variety), Eupatorium adenophorum , 
E. Raffilli , Choisya ternata , Aucuba jap'onica (small and large-leaved forms), 
and Veronica speciosa (variety). Of these, the most useful was found to be 
Eupatorium adenophorum. This plant is easily propagated from cuttings 
and can be kept healthy throughout the winter in a cool greenhouse. It 
is suitable for transpiration experiments, because its rate of water loss is 
sufficiently high to be easily measurable, and is convenient for porometer 
experiments, owing to the ease with which the leaf chamber can be fixed 
to the leaf, which is neither hairy nor unduly smooth. The stomata are 
confined to the under surface of the leaves and are rather small, but are 
quite active in their response to changes of illumination. 
My thanks are due to Mr. W. Hales, Curator of Chelsea Physic Garden, 
who has, throughout this work, maintained for me an ample stock of the 
necessary plant material. 
Experiments have been carried out, using potted plants and also cut 
shoots mounted in a potometer. In the former case the pot was enclosed 
in the usual aluminium sheath and the top covered with rubber sheeting. 
The whole was placed on a small retort stand and the cups for the 
potometer were held in clamps. The retort stand was weighed with the 
plant on a large balance. 
In the experiments with cut shoots, the potometer was made from 
a small burette or graduated pipette which was sealed to one arm of a U- 
shaped tube. The other arm of this was attached to the cut stem of the 
plant by a short piece of rubber pressure-tubing, which kept the shoot 
upright. The curved portion of the U-tube was fastened to a wooden 
stand, to obviate the need of clamping in an upright position. Porometer 
cups were held in holes in flat pieces of wood (garden labels answer the 
purpose well), which were wired to screw-clips gripping the burette or 
