462 The American Naturaitst. [May, 
ON THE GROWTH-PERIODICITY OF THE POTATO- 
TUBER.’ 
BY CONWAY MACMILLAN. 
HILE considerable research has been bestowed upon the 
physiology of bulbs, corms, and tubers, it does not appear 
that any extended observations have been made upon the method 
of. growth of such an organ as the potato-tuber. It is a well- 
known fact that the growth in length of upright stems and other 
aérial organs is not regular, but exhibits a marked daily perio- 
dicity, the time of greatest average growth being in most cases not 
far from six o'clock in the morning. Upon this subject, since 
the researches of Sachs,? Baranetski,? Pfeffer and others, & 
number of observations have been made by various investigators. 
‘It appears that in most above-ground organs there is a clearly 
marked diurnal period, unless this period is obliterated by etiola- 
tion, suffocation, anzesthesia, or some other abnormal condition. 
We know, too, 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 more rapidly or continue in it longer 
proportionately than other organs or similar organs on the other 
. plant, or in the same plants under different conditions. The gr owth 
in length, then, of any organ is not regular, but is to be grap 
cally represented as a wavy curve, with an ascending portion, @ 
climactic portion, and a descending portion. In all of the pe e 
this large curve, the climax of which represents the grand-perio 
of growth, one must notice the rhythmic pulsations due to a 
daily growth-periods, and more or less synchronous with the 
alternating periods of light and darkness, of higher ‘and lower 
temperature, of less and of greater oxidation. 
1 Read before the Minnesota Academy of Science, May 5th, 1891. 
? Arbeit. d. Wiirtzb. Institute, 1873. 
3 Die tägliche Periodicität d. Langenwachsthums, 1879. 
t Physiolog. Untersuchungen, 1873. 
