360 Grozvth-Pcriodicity of the Potato Tuber. 



lower part of the main stem was living, green tissue still to be 

 found. During the whole twenty-four hours little divergence of 

 the tracings from the horizontal could be seen, but during the suc- 

 ceeding twelve hours a slight drop began. At 7 a. m. of the sev- 

 enth day a decided drop began continuing until 11 a. m. There 

 then succeeded a period of gradual dropping which disappeared 

 about 3 p. m. Another drop took place in the evening from 6 to 

 9 p. m. The eighth day began with a drop at 7. a. m., continuing 

 until II, when three hours of horizontal marks followed. At 2 p. 

 m. a five-hour drop began, and continued as a gradual depression, 

 until 10 a. m. At 7 a. m. another abrupt drop took place, termin- 

 ating at 1 1 :30 a. m. At 3 p. m. a gradual drop lasting until 8 p. 

 m. followed. During succeeding days the same rhythm contin- 

 ued, only the drops became slighter and slighter. Finally the 

 needle ceased to trace. The explanation of these very curious 

 maxima and minima in the growth of the tuber is a complicated 

 matter. It can be given as yet only conjecturally. Before pass- 

 ing to any such conjectures, it may be well to give the conclusions 

 arrived at in the experiment described above : 



(a) The increase in diameter of the potato-tuber is not reg- 

 ular but is rhythmic. 



(b) Maxima of growth are not of long duration, and are 

 followed by periods of slower growth or of entire absence of 

 growth. 



(c) Maxima of growth may occur either once or twice and 

 perhaps oftener during twenty-four hours. 



(d) The maxima of some days are greater absolute maxima 

 than those of other days. This indicates a grand period for the 

 tuber. 



(e) Regular periodicity in the tuber continues after the 

 periodicity of the aerial stem is lost. 



(f) Connected with profound changes of condition in the 

 aerial stem changes in the periodicity of the tuber may be noted. 



(g) There is some connection between the periodic growth 

 of the tuber and the periodic growth of the aerial stem. What 

 this connection is does not appear. 



(h) There is also, it is probable, an independent periodicity 

 in the growth of the potato-tuber which is obscured and modified 

 by the secondary induced periodicity which is connected with the 

 aerial-stem conditions and mode of growth. 



