566 BACTERIAL DISEASES OF PLANTS 



hunger, which may result, as I have shown, from various causes, 

 parasitic and non-parasitic. 



Thus, whether brought about by direct abstraction of water, 

 as through wounds; or by dry conditions, as in the potatoes on 

 my table; by semi-asphyxiation, as under obturated lenticels; 

 by growth of wet tissues in a minimum of very moist air, as in 

 my sealed tubes; by direct addition of ammonia, formaldehyd, 

 or acids from without, as through stomata; or finally, by the in- 

 troduction into the cells of an alkali and acid-forming tumor 

 parasite, the end result is the same, viz., the formation of a 

 tumor, the type of which (hypertrophy or hyperplasia) depends, 

 I think, on the extent of the cell-stimulus, or cell-paralysis; 

 and the extent of the growth on whether the stimulus is self- 

 limited or is due to a continually multiplying parasite. 



Recently light has been thrown on some of these problems by 

 Dr. R. B. Harvey, of the U. S. Department of Agriculture. In 

 experiments on the hardening of plants by cold he has shown that 

 when cabbages are exposed to — 3°C. for about 30 minutes their 

 leaves freeze in spots and intumescences afterward develop 

 from these spots. Such frozen spots extrude water into the inter- 

 cellular spaces, as shown by the evanescent spotting, and judging 

 from his experiments on egg-white plus phenolphthalein and on 

 leaves of Coleus, using their cell-sap as an indicator, freezing 

 increases the acidity, or hydrogen-ion content, of such cells. 

 This leads to an increased cell-permeability and to the presence 

 in the tissues of an excess of sugar and an excess of peroxidase, 

 all of which favor growth. 



In experiments made with red cabbages I could not get the 

 same result that Dr. Harvey obtained with Coleus. The frozen 

 leaves showed no change in color. Because all of the red pig- 

 ment is in the surface cells, the leaves were then ground, mixed 

 with a measured volume of distilled water and frozen for three 

 hours, but again there was no change in color. All of the check 

 tubes but one were kept at room temperature and did not change 

 in color. One check tube was exposed for the same length of 

 time (three hours) in warm water (60°-62°C.) and this one 

 reddened decidedly. On freezing the fluid in this tube changed 

 color plainly but it became bluer not redder. 



