miscellaneous: stimuli underlying tumor-formation 531 



because, as we have seen, intumescences often occur in hot- 

 houses where there can be no question of diminished oxygen 

 (I mean, of course, that external to the plant) or of increased 

 external carbon dioxid. The active factors in the production 

 of these tumors must then be diminished internal oxygen and 

 increased acidity of the tissues (as shown by titrations), both due 

 to the saturation of the ambient air which prevents transpiration 

 (which normally by continual movement of absorbed water 

 continually bathes the tissues in an aerated fluid) and to water- 

 logging of the tissues, which prevents the entrance by way of the 

 stomata of sufficient external air for the needs of respiration. 

 There is certainly saturation of the atmosphere in my sealed 

 tubes as there may be in defective greenhouses and at the same 

 time there must be consumption of the oxygen by respiration 

 and increased absorption of water by the lower parts of the 

 plant (roots in hothouse plants and living cells at the base of the 

 raw potato blocks in my sealed tubes). This absorbed water, 

 carrying its dissolved air, is passed on to the shoots rather in 

 excess tending to internal saturation and turgor pressure, 

 but moving more and more slowly owing to the interrupted 

 transpiration. The tissues, especially under the stomata to- 

 ward which all these various streams of water are moving, will 

 then be very full of a sluggish current from which the insufficient 

 oxygen is quickly removed, and hyperplasia must then set in 

 as a compensatory measure, or otherw'se there will be asphyxia- 

 tion of the tissues. As a matter of fact, a little later, suffoca- 

 tion did occur in all the roots and shoots developed in my sealed 

 tubes (Figs. 402, 404, 406, 408 D, E) but the death of parts, as 

 I have said, is a subsequent matter, an end term, which does not 

 concern us here. The reason the hyperplasia sets in only under 

 the stomata or lenticels is because here are the youngest most 

 sensitive cells. 



After the above was written I discovered that intumescences 

 could be produced on potato shoots in two other ways: (1) by 

 bacterial destruction of the tops while the roots continue to 

 function, and (2) by reducing the intake of water. 



On April 10, 1919, I made inoculations on potato plants 

 with Bacillus phytophthorus for another purpose (Fig. 211) and 



