185 



and from the axil of one of these leaves a sprout, about 5 mm. 

 long, has developed. The tuber itself is about 25 mm. long and 

 20 mm. wide at the thickest part. 



This specimen is instructive, not only as a unique demonstra- 

 tion of the stem nature of the " potato," but also in connection 

 with a recent theory that potato tubers are caused by a fungus. 

 It is known that a species of fungus, a Fiisariiun, is endotrophic 

 with Solaniim tuberosiivi, and the question has been raised by 

 Bernard * as to w^iether or not the tubers are an effect of this 

 fungus, and dependent upon it for their formation, Jumelle f 

 infected soil artificially with a species of Fusaritan, and planted in 

 the infected soil 5. Coniuiersoni, a species that forms only small 

 and few tubers. A control planting was made in sterilized soil. 

 The infection produced no constant results, however, with 

 Solanum Commersoni, and contradictory results with varieties of 

 5". tuberosum. In the case of the " Early Marjolin " variety, asso- 

 ciation with the fungus was accompanied with increased tuberiza- 

 tion, while the opposite result was obtained with the " Geant de 

 Lyon " variety. 



The formation of a tuber in the air and light, therefore, is of 

 interest as showing that tuber-formation may take place under con- 

 ditions in which the Fiisariiun, in all probability, does not enter 

 as a factor. 



Of potato tubers Goebel % says : " They are nothing else then 

 than leaf-shoots which on account of their position in the whole 

 shoot-system of the plant have become accustomed to an under- 

 ground life, and subsequently under the influence of the material 

 supplied from the aerial leafy shoots have become transformed 

 into tubers." The phenomenon is an illustration of what Goebel 

 calls " qualitative correlation." 



It is instructive to recognize just what has taken place in the 

 present anomaly. If we disregard the small amount of photo- 

 synthesis that may have occurred, owing to the slight develop- 



* Bernard, Noel. Etudes sur la tuberisation. Rev. Gen. de Bot. 14 : 139, 269. 

 1902. 



f Jumelle, Henri. De I'influence des endophytes sur la tuberisation des Solanum. 

 Rev. Gen. de Bot. 17 : 49. 1905. 



j Goebel, K. Organography of plants. Eng. trans, by Balfour, i : 215. 1900. 



