Apr.22, iQis Intumescences 257 



pustule projected from the leaf surface to a height equal to three or more 

 times the thickness of the leaf (PI. 19). When the enlarged cells were 

 exposed to desiccation, however, the protrusions w^ere much less promi- 

 nent, as would be anticipated. Humidity had previously been reported 

 by the writer as exerting a similar effect in the production of cankers on 



Citrus spp. {18). 



DISCUSSION OF RESULTS 



It must be admitted that the development of intumescences upon 

 young cabbages following the violent projection of sand against them 

 came somewhat as a surprise, even though wind-blown sand was sug- 

 gested to the grower as the probable cause when the field diagnosis was 

 made. Mechanical injury appears not to have been previously observ^ed 

 to be an ultimate cause of the formation of intumescences on plants. 

 The complete chain of events connecting the stimulus of wounding at 

 the one end and the mature intumescence at the other is, of course, not 

 completely known. However, it is believed, in the light of the theory 

 of edema as advanced by Fischer (j), that an adequate explanation can 

 be offered for the formation of the enlarged cells. The exposure of the 

 protoplasts following the rupture of the epidermis from the impact of 

 the particles of sand would certainly be followed by oxidative changes 

 within these protoplasts. This assumption is based on the fact that 

 Von Schrenk (8) found an increase in the oxidizing enzyms in intu- 

 mescences on cauliflower {Brassica oleracea hotrytis) leaves and upon our 

 general knowledge of oxidations following exposure of tissues through 

 wounding. The acids produced from these oxidations would not only 

 increase the hydration capacity of the emulsion colloids within the 

 wounded cells but would also involve adjacent cells. The fact of a 

 heightened affinity of colloids for water following even a slight increase 

 in acidity is a well-known property of colloids. This would be followed 

 by the distention of involved cells to a point where equilibrium had been 

 established, a condition determined by such factors as differences in 

 adsorption, solubility, and chemical combination of the cell colloids 

 themselves. 



This explanation, while it may not meet with approval by students 

 of overgrowths, is attractive because of its simplicity and because it 

 accords with established facts concerning absorption by hydrophylic 

 colloids. It is furthermore believed that the theory of absorption can 

 be applied to the experimentation presented by other investigators and 

 can also be employed to explain adequately the mechanism of other 

 types of overgrowths in plants such as knots, tumors, and galls of insect, 

 nematode, bacterial, and fungus origin. 



SUMMARY 



(i) Various ultimate and proximate causes have been assigned for the 

 formation of different types of overgrowths as intumescences, cankers, 

 knots, tumors, and galls present on plant parts. 



