522 BOTANICAL GAZETTE [December 



of the sprout, but no case of penetration was observed. At the 

 writer's suggestion, Miss Lucile Keene, 2 then a senior in the 

 University of Missouri, endeavored to secure the penetration of one 

 potato by the sprouts of another. The two tubers were bound 

 tightly together with stout cord, and buried in moist sphagnum. 

 In no case were the sprouts successful in penetrating through the 

 uninjured skin of the adjacent tuber, though the latter was indented. 

 However, if the sprouts were placed against the cut surface of 

 another potato, they entered easily, made their way through the 

 parenchymatous tissue to and through the skin at the farther sur- 

 face. That the sprouts can easily penetrate the cortex from 

 within, though not from without, was shown by the abnormal 

 cases where such penetration was general (fig. 4). Had enzy- 

 molysis been a factor, the sprouts might as easily have penetrated 

 from without. The experiments with roots, as noted above, gave 

 analogous results. 



The observed facts will not admit of a definite denial of reversal 

 of polarity. Many of my dissections cannot reasonably be inter- 



seems 



always probable, that the ingrown sprout arose as a lateral branch 



>/ 



tuber. 



make 



on pure faith as one involving the conception of a reversal of polar- 

 ity in the shoot. Detmer (2) long since called attention to the 

 fact that while the intensity of polarity varied greatly with different 

 species, it was especially well marked in the case of Solatium 

 tuberosum. 



The cause of tuber-formation 



What caused some of the ingrown sprouts to form tubers? 

 The answer to this question is of course bound up with the larger 

 question as to why potato branches ever tuberize. The literature 

 bearing on this problem was discussed by the writer (3, 4) in 1906, 

 and need not be reviewed here. Reference may again be made, 

 however, to the suggestion of Bernard that the formation of tubers 

 is due to a species of Fusarium, endotropic with Solatium tuberosum. 



3 Miss Keene also found a similar case of ingrowing sprouts in Montana, but 

 the sprouts were only slightly branched, and formed no tubers within the old potato. 



