Growth Studies. 
III. A ‘ Volumometer ’ Method of measuring the Growth of Roots. 
BY 
J. H. PRIESTLEY and W. H. PEARSALL. 
With one Figure in the Text. 
O NE of the outstanding difficulties of studying growth in plants is the 
making of continuous measurements upon a single plant grown 
under controlled external conditions. In studies of animal growth this 
difficulty does not arise, since it is a relatively easy matter to w,eigh the 
animal at stated intervals. In plants, however, measurements of total 
weight are of doubtful value, owing to the large percentage of water present 
(about 90 per cent, usually), which may vary without reference to any 
actual change in protoplasmic mass. It is, moreover, difficult to weigh fresh 
plants accurately without such precautions as must destroy the plant as 
a growing individual, and of course dry weight determinations involve the 
entire destruction of the individual. In addition, even length measurements 
become practically impossible when dealing with branched structures such 
as older roots, and their value must always remain slight, on account of 
variations in the thickness and structure of the organs at different stages 
of development. 
It was an attempt to overcome some, at least, of these difficulties that 
led the authors to devise an apparatus for measuring the volume of roots 
at different stages of growth. The essential idea involved in this apparatus 
was that roots should remain in a fixed position in some container which 
would allow of their being grown under suitable conditions for a com- 
paratively lengthy period. The container had, therefore, to provide for 
the accurate measurement of small volumes, and yet had also to be of 
considerable size to permit extensive root development. In addition, it 
had to be provided with a means of renewing the nutrient solution. 
In the following paragraphs an account is given of an apparatus devised 
to meet these conditions and used with satisfactory results. 
The container (D C in figure) consisted of a glass vessel holding about 
300 c.c. of water, with a narrow neck of about 2-5 cm. diameter. The neck 
was proyided with a blue poicelain point, directed downwards. The 
[Annals of Botany, Vol. XXXVI. No. CXLIV. October, 1922.] 
