MISCELLANEOUS: TUMORS IN ABSENCE OF PARASITES 489 



of water from the injured cells and this leads to a change in color, 

 but this color difference soon disappears and the leaves appear to 

 be normal until the growth has advanced to the second or third 

 da}'. As I have recorded elsewhere ("Mechanism of Tumor 

 Growth in Crown gall"), the same transient spotting precedes 

 the development of tumors on cauliflower leaves exposed to 

 dilute vapors of ammonia (Fig. 381) and is undoubtedly to be 

 explained in the same way, as due to loss of w r ater from injured 

 cells into the surrounding intercellular spaces. 



Earlier in the same year (1918) Wolf showed that intumes- 

 cences may be produced on cabbage leaves by means of a sand- 

 blast and ascribes intumescences occurring naturally on cabbage 

 plants in the field to sand driven by the wind. In this he 

 is unquestionably right. They may also be produced on cauli- 

 flower leaves in the hothouse by sandpapering the leaves, 

 and on bean plants by various woundings. All of Wolf's figures 

 are hypertrophies (Fig. 382). 



Some years ago (1892-93) George F. Atkinson experimented 

 with the oedema of the tomato, which is an intumescence (Fig. 

 383) and reached the conclusion that on susceptible varieties 

 (Fig. 384) oedema may be induced by insufficient light and bad 

 ventilation coupled with too much water in the soil, and a soil 

 temperature too near that of the air, leading to the accumu- 

 lation of acids in the plant and to weak cell-walls, easily stretched 

 as water is imbibed. He says: "When there is an abundance 

 of water in the plant these acids draw large quantities into 

 the cells, causing the cells to swell, resulting many times in 

 oedema." . . . "Ordinarily there is no increase in the number 

 of cells." He claims to have produced oedema by forcing an 

 excess of water into the plants, but his experiments were few, in 

 a place where oedema was naturally very prevalent, and they 

 should be repeated. If he made any experiments to determine 

 increased acidity they are not mentioned. 



Sorauer, who, following earlier writers, gave the name of 

 intumescences to these wart-like growths which occur at times 

 on plants of many species, thought they were "caused by an 

 excess of water during a period of low assimilation. '' In 

 another place he speaks of these formations as due to "an 



