5i8 



BOTANICAL GAZETTE 



[DECEMBER 



toward the surface of the potato and penetrated the parent tuber 

 from without. But it is only the a-priori improbability of a reversal 

 of polarity that makes the writer hesitate to declare it. The evi- 

 dence clearly indicates it, but the evidence is not all in. 



Fig. 6. — Sections through sprouting tubers of Solarium tuberosum: at the left, a 

 normal "eye"; at the right, an "eye" with sprouts ingrown and indicating a reversal 

 of polarity of the branch. 



Discussion 



What caused the ingrowing of these snrouts it is difficult 



say 



room 



much drier than that of cellars, where 



kept, suggests that the low humidity was at least a contributory 

 cause. But equally interesting is the question of how the sprouts 

 made their way through the tuber. Were the channels through 



which they grew formed by the digestion of the potato tissue by an 

 enzyme secreted by the tip of the sprout; or did the channels result 

 merely from the mechanical pressure of the sprout as it elongated? 

 It will be recalled that a similar problem arose in connection 

 with the endogenous emergence of lateral roots. Considerable 

 difference of opinion has existed on this question. The first 

 physiological study of the emergence of lateral roots appears to have 

 been made in 1871, by Reinke {9), who stated that the channel 



