1891.] The Growth-Periodicity of the Potato-Tuber. 463 
Seasonal rhythm in the growth in girth of organs is well 
known in the ordinary woody stems of Dicotyledons® and 
Gymnosperms, where the increasing tensions of later months 
reduce the rate of growth below the rate of the earlier months. 
This periodicity is a more simple and readily explained form than 
those forms which have been alluded to above. It is found 
principally in organs provided with a cambium cylinder and a 
relatively inextensible bark, and is referred to merely by way of 
illustration. While the potato-tuber, which is to be considered, 
has a cambium area, it can scarcely be said to have a cortical 
area at all analogous to that of the erect tree-trunk, We shall 
not find the tuber, protected as it is and growing during a single 
Season, affected by the conditions of alternate freezing and 
thawing, wind disturbance, stress, flexion, etc., which have so much 
to do with seasonal periodicity of growth in girth of woody stems. 
A few months ago the writer was struck with what seemed to 
be a great dearth of investigations into the manner of growth of 
tubers, and forthwith gave some attention to devising a method 
by which the gap in our knowledge of tuber-physiology might 
be filled in part. After due deliberation a method was formulated 
and applied, with but imperfect success at first, but as experience 
became wider the imperfections were gradually remedied. In all 
of the experiments Mr. C. P. Lommen, student in biology at the 
University of Minnesota, gave much assistance in setting up appa- 
ratus, and by one or two helpful suggestions concerning certain 
technical difficulties which presented themselves in the course of 
our investigations. The method of research first adopted by us 
has been described elsewhere somewhat in detail? but upon this 
method certain important improvements have been made. The 
apparatus used was the Baranetski self-registering auxanometer, 
with electric clock attachment, manufactured by Albrecht, of 
Tiibingen. At first both wheels of the apparatus were not 
employed, but afterwards it was found that twe wheels could be 
combined in such a way as to multiply the tracings tenfold, and 
‘Pfeffer. Pflanzen-physiologie, IT., 89. 
*Hartig. Anat. und Phys. der Holzpflanzen, p. 366. 
"L.c. Botan, Gazette, May, 1891. - 
