1891.] The Growth-Periodicity of the Potato-Tuber. 469 
another, so that the periodic growth of the above-ground stem 
might induce a periodic growth in the below-ground tuber. 
4. The asynchronous grand-periods of growth of the different 
above-ground organs might be reflected in an irregular and erratic 
periodicity in the below-ground tuber. 
5. Combinations of these various conditions, and : a modifica- 
tion of them all by the independent rhythm of the tuber itself, 
would have to be considered, and only by the most elaborate and 
extended researches could the proximate causes for the observed 
tuber-periodicity be detected. 
In closing this contribution to the physiology of tubers, one 
word may be added by way of note. It is possible, as may be 
shown, to apply auxanometer methods to root stocks by uncover- 
ing the root stock, attaching a silver thread, running it horizon- 
tally to the open side of the box, passing under a horizontal roller 
and upward, and finally adding the linen or silken thread which 
runs on the small circumference of the first wheel. Or, in this 
case, doubtless one wheel alone could be employed. Such study 
of underground stems, as in the grass root stock, the potato rhiz- 
ome, or any other underground stem, would scarcely fail to throw 
some light upon the method of growth of the tuber. A compari- 
son of underground organs should be made along this line. 
University of Minnesota, Minneapolis, May rst, 1891. 
