356 Grozvth-P eriodicity of the Potato Tuber. 



by etiolation, suffocation, anaesthesia, or some other abnormal con- 

 dition. Upon this subject since the researches of Baranetski 

 and Pfeffer much attention has been bestowed, and we know 

 that besides the daily periodicity there is a grand period of growth 

 for each organ of the plant, that some organs reach the grand 

 period of growth more rapidly or continue in it longer proportion- 

 ately than other organs or similar organs in other species or in the 

 same species under different outward conditions. The growth in 

 length of any organ therefore is not regular, but it is to be graphi- 

 cally represented as a wavy curve with an ascending portion, a 

 cHmacteric portion and a descending portion. In all of the parts of 

 this great area, the climax of which represents the grand period 

 of growth, one must notice the rhythmic pulsations due to the 

 daily growth period, and more or less synchronous with the alter- 

 nating periods of light and darkness, of higher and lower tempera- 

 ture, of less and of greater oxidation. 



Seasonal rhythms in the growth of girth of organs is well 

 known in the ordinary woody stems of Dicotyledons and Gymno- 

 sperms when the increasing tensions of later months reduce the 

 rate of growth below the rate of the earlier months. This period- 

 icity is a more simple and readily explained form than those 

 periodicities which have been alluded to. It is found principally 

 in organs provided with a cambium cylinder and a relatively in- 

 extensible 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 anal- 

 ogous 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-disturbing, 

 and so forth, 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 the entire ab- 

 sence of investigations into the manner of growth of tubers, and 

 gave more attention, forthwith, 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 ap- 

 plied, Avith but imperfect results at first; but as experience be- 

 came 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 



