Grozvth-Pcriodicity of the Potato Tuber. 361 



With reference now to the conjectural explanation of the 

 periodic growth of the potato-tuber, very little can be expected at 

 this stage of the investigation. Whether like embryonic plants of 

 Hedera, with their heliotropic irritability, the potato-tuber retains 

 somehow, in hereditary fashion, its above-ground periodicity and 

 thus gives hint of the time when its precursors were exposed to 

 rhythmic alternation of light and darkness, is entirely an open 

 question. On the other hand it is equally uncertain whether the 

 induced periodicity is due to one or many causes. Some of the 

 lines of research are indicated below and it is hoped that they will 

 be followed to their rational conclusion. 



(i) The rhythm of assimilation in the above-ground stem 

 may affect the growth of the below-ground tuber. The synthesis 

 of carbohydrates is a diurnal affair. From these carbohydrates 

 the substance of the tuber is formed. Thus the rhythm above 

 might induce a rhythm below. 



(2) The conversion of plastic materials into reserve materi- 

 als is characteristic of an organ like the tuber. This conversion 

 depends upon the activity of certain ferments which are results of 

 destructive and constructive metabolic changes in the shoot-area. 

 These metabolic changes are connected with the respiration- 

 function and this is a periodic or rh3^thmic function. 



(3) The growth of the above-ground stem is strongly peri- 

 odic and demands, in any plant, the same kind of material which 

 would be supplied to a growing tuber. This drain upon the 

 plastic-material in one direction might induce a corresponding 

 dearth of it in 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 dif- 

 ferent 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 modi- 

 fication 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 by way of note may be added. It is possible to apply 



