NOTE. 
A SIMPLE ROOT AUXANOMETER — None of the standard methods for 
measuring the rate of growth of roots is convenient for use with large practical classes 
in that they involve either the use of a horizontal microscope or other expensive 
apparatus, or else the magnification of the movement is not sufficient to allo w the rate 
of growth to be determined in the course of a few minutes. 
A simple method has been used in this laboratory and has proved entirely 
satisfactory for class purposes. It is sufficiently simple and straightforward for use 
by elementary students, while, with certain modifications, it can be made to yield 
results of such accuracy as to warrant its employment for more serious purposes. 
In its simplest form the apparatus required consists of a gas jar, a, fitted with 
a cork, b. Through a hole in the cork a glass rod, c, runs freely. A long pin passes 
through the cork and impales a seedling, d, with a straight root. 
The method of procedure is as follows. The glass rod having been inserted 
through the hole in the cork, as shown in the sketch, a convenient quantity of water is 
poured into the jar and the height of the seedling adjusted by means of the pin until 
the root tip just touches the surface of the water. Upon lifting up the glass rod 
a short way, the level of the water in the jar falls and leaves the root tip above the 
surface. The rod is then slowly lowered until the root tip just touches the water 
surface, and the position of the rod in relation to the top of the cork through which 
it passes (or to a wire pointer) is recorded by making a mark upon it. The apparatus 
is left for 5 or 10 minutes and a fresh reading taken *. this will be found to differ 
from the previous one by 2 or 3 cm. or more of the glass rod in the case of an actively 
growing root. 
The exact moment at which the root tip touches the water surface is very 
evident, owing to the sudden rise of the water round the root by surface tension^ — it is 
much more evident than the moment of contact of a metal pointer with a surface of 
mercury which does not ‘ wet * it. If the glass rod is lowered slowly and carefully, 
readings of its position should vary less than a millimetre. 
The magnification involved can be determined by measuring the inside area of 
the cross-section of the jar (A) and the area of the cross-section of the glass rod (s ) : 
the actual elongation of the root will be the distance apart of the marks on the glass 
rod X — . For example, with a glass rod 5 mm. in diameter and a jar 5 cm. in 
o 
diameter, the magnification will be 100 times, and every millimetre the glass red is 
lowered will cause the water level to rise o«oi mm. in the jar. It is a convenience to 
employ a rod graduated in millimetres : the absolute* amount of growth of the root, if 
required, is then obtained by dividing the reading on the glass rod by the magnifica- 
tion coefficient. 
[Annals of Botany, Vol. XXXIV. No. CXXXVI. October, 1920.} • 
