miscellaneous: stimuli underlying tumor-formation 555 



The further discussion of the cell response may be left until we 

 have considered other cases. 



If cauliflower plants are exposed to vapor of alcohol carrying 

 acetic acid, or are sprayed lightly with acetic acid water (1 

 part of acid to 100 or more parts of water) tumors begin 

 to form at once in many parts of the leaf, if it is not too old. 

 Here again the entrance of the stimulating substance is through 

 the stomata and as a result we may assume, for the present at 

 least, that always there is a temporary partial paralysis of the 

 protoplasmic membrane, that there is initial loss of water, 

 that cell-acidity is increased, that the mechanism of respiration 

 and transpiration is disturbed, that consequently, cell-division 

 is forced, with consumption of sugars, amino acids and proteids, 

 and that subsequently there is a compensatory movement of 

 water and food-stuffs into the area from surrounding tissues, 

 the result being the rapid development of a hyperplasia. This, 

 however, grows only for a week or two, just as in case of the 

 ammonia tumors, because there has been no repetition of the 

 initial small stimulus such as is continually taking place in crown 

 gall and in other tumors due to parasites. If it were possible to 

 repeat the stimulus at frequent intervals, undoubtedly large 

 tumors would result. 



Although I have studied sections of a good many acetic 

 acid tumors I have not seen any evidence of hypertrophy except 

 in the earliest stages in cells nearest to the stoma and likely to 

 have received the greatest volume of the acid. Always, the 

 remoter growth is a hyperplasia, as in the examples shown (Figs. 

 368 to 372). I have followed the sections in series, from the 

 level of the leaf surface down to the bottom of tumors, and 

 immediately under them in the depths of the leaf, without 

 finding any evidence of killed cells in the deeper parts, but 

 always, it seems to me, there is at least some slight evidence 

 of injury at the surface of the tumor in its middle part close 

 under the epidermis (Fig. 378) . The initial growth of the tumor 

 is not from the epidermis itself but from cells immediately 

 below it. Tumors are more apt to begin in the lower parts of 

 the leaf than in the upper parts, possibly because the stomata 

 on the lower surface are more apt to be open and are also more 



