miscellaneous: stimuli underlying tumor-formation 541 



of the intumescences, but only a diminished oxygen suppl}^ due 

 to stasis of the water current and presumably an increased acid- 

 ity of the tissues due to this fact, probably in part also to pres- 

 ence of acid by-products of the parasite; and this is illuminating 

 as to the origin of other hyperplasias. 



Finally, intumescences occurred freely under stomata on 

 swollen stunted leafless shoots (Fig. 411) developing in April from 

 dry potato tubers kept on my laboratory table at 25°C., part in 

 open deep glass jars exposed to a north light and the remainder, 

 of another variety, enclosed in covered paper boxes. Here again, 

 neither high temperature, nor bright light, nor feeble light, nor a 

 saturated atmosphere, nor in this case excessive water supply had 

 anything to do with their production. In these shoots we know 

 that there was a defective transpiration apparatus and we may 

 assume that there was a sluggish, feebly aerated water-current, 

 and consequently that there was more or less oxygen-hunger. 

 These swollen shoots were rather hard and were gorged with 

 starch (Fig. 412), showing that there was in them, as there is often 

 in crown galls, a great excess of sugar beyond that needed for 

 growth. This was also made evident by tests with Fehling's 

 solution which showed reducing sugars to be scanty in the flesh 

 of the mother tubers, but to be very abundant in the shoots 



A. Pediceled, white-shining, smooth tumor covered by an epidermis. It 

 arises from the cambium. Photo. March 22. X 2, nearly. On May 2 there 

 were also several small tumors at t. 



B. The membrane has ruptured widely across the middle of the tumor from 

 internal pressure and a tiny shoot has started from its base in the vicinity of the 

 pedicel. Photo. March 31. 



C. Further development of the shoot. The tip of the shoot is green, the roots 

 are white and the remainder is pink. The tumor is yellowish white. The sealed 

 tube has been, all of the time, in the dark at 23°-2.5°C. The beginnings of in- 

 tumescences are visible on the pink and green parts of the shoot. Photo. April 9, 

 1919. 



D. E. Front and back view of Fig. C, three weeks later. Asphyxiation has 

 l^egun. The intumescences are now distinct. Under D there is a freshly cut 

 surface; at X, the newly formed cork-layer is visible. Photo. May 2, 1919. X 5. 

 The tissues were not tested for acidity, but the juice of similar intumescent shoots 

 from other sealed tubes in this series was strongly acid (+75 on Fuller's scale) and 

 full of starch. The normal acidity of potato tubers is + 20. 



