284 Delf. — Studies of Protoplasmic Permeability by 
to van Rysselberghe, the effect of temperature is accelerating only up to 
20 0 C., but this conclusion is open to criticism both on experimental and 
on theoretical grounds, as will be shown in the course of this paper. 
It was suggested to me by Dr. F. F. Blackman that the plasmo- 
lytic method of investigating the effect of temperature on the permea- 
bility of protoplasm to water could be made more effective by the use 
of seme magnifying arrangement enabling the observer to follow the 
process in all its stages. This has been achieved by means of a particular 
form of optical lever devised by him, which gives a magnification of 
350 diameters on changes in length of short strips of tissue, which though 
fixed in position are not subjected to any strain. I am indebted to 
Dr. Blackman not only for the use of this apparatus, but for much help 
and suggestion throughout the research. The work was carried out in 
the Cambridge Botany School in the year 1915, while holding a Yarrow 
Research Fellowship at Girton College. 
Section II. Apparatus and Regulation of Temperature. 
The method of measuring the rate of tissue-shrinkage is based upon 
the great magnification of alterations of length of strips of tissue that can 
be obtained by the principle of the ‘ optical lever V The short lever 
C, which the plant actually displaces, carries a tiny mirror on which the 
image of a cross wire is projected by a Nernst burner (v, Fig. 1) in a 
tube. From the mirror the image is reflected on to a millimetre 
scale W, and a magnification of x 350 is thus attained. The fulcrum of the 
optical lever is a small aluminium cylinder free to rotate on a horizontal 
axis, the bearings being steel points working in agate cups. From the 
cylinder adjustable wires project at right angles to the axis for a few 
mm., and to these the threads connecting to the plant tissue below on 
one side and to the counterpoise D below on the opposite side are 
attached. The counterpoise is a few mm. of fine wire and so adjusted 
that it will just assure a downward movement of its own side when the 
thread connecting to the plant on the other side is cut. There is thus 
always a* minute extension-strain on the plant tissue. The clamp for the 
plant below is carried on the same ‘ invar ’ rod, E, as the lever, and the whole 
is supported from above independently of the rest of the apparatus. 
The clamp is designed to hold a tubular structure such as part of an 
inflorescence axis or a cylindrical leaf, so that water or solution may 
be passed through it continuously, and bathe the delicate pith cells which 
line the interior. The plant cylinder A (Fig. 1) is firmly fixed to a glass 
nozzle G of narrow bore, by winding it round with cotton fastened below to 
a small rectangular block of cork through which the nozzle passes. The 
1 The principle of the optical lever has been previously used by Professor Bose for records of 
minute movements of plants; see J. C. Bose: Plant Response, 1906. 
