igi2] GAGER— INGROWING SPROUTS 523 



This suggestion was tested experimentally by Jumelle, but with 

 negative results. Quite evidently this factor does not enter in the 

 tuberization of branches of the ingrown sprouts. Goebel cites 

 tuber-formation as an illustration of "qualitative correlation/ 7 

 stating that it is a function of the relation of the branch to the whole 



shoot system, its underground position, and the material supplied 

 to it. 



It seems to the writer that an attempt to explain such a deep- 

 seated character as the formation of tubers by potato plants is like 

 trying to explain the cornness of corn. We may ascertain experi- 

 mentally what external conditions or combination of circumstances 

 must be realized in order that tubers may result, but that they 

 form at all is because the plant is Solatium tuberosum, rather than 

 Pisum sativum or Solatium Dulcamara. To use a recent and very 

 valuable terminology, the formation of tubers by Solatium tubero- 

 sum, or by any other tuber-forming species, under suitable environ- 



» 



mental conditions, is an expression of the genotypical constitution of 

 the plant. Further explanation than this lies far in the future. 



Conclusion 



anum tuberosum make their way 



not by enzymatic digestion of a 



mechanical pressure which accom- 



panies growth in length. 



sufficient 



from 



from 



3. A reversal of polarity in ingrowing 



definitely demonstrated, but there is evidence pointing to this 

 conclusion. 



4. Tuberization of branches takes place freely on ingrown 



sprouts of Solatium tuberosum. 



5. The formation of tubers on Solanum tuberosum is a function 

 of external conditions plus the genotypical constitution of the 

 species. 



The Brooklyn Institute of Arts and Sciences 



Brooklyn Botanic Garden 



