440 SMITH— MECHANISM OF OVERGROWTH IN PLANTS. 



growths which I think I am warranted in designating as incipient 

 crown galls. The overgrowths I have obtained are small, as was 

 to be expected from the application of a single slight stimulus. 

 They do not continvie to grow because they are the response to an 

 abnormal outside influence of very limited duration, or to put it in 

 another way, because there is no parasitic organism back of the 

 growth, as in the case of the natural crown gall, to continually 

 stimulate it by means of its excretions. In this particular, that is 

 in the continuous slow introduction of these substances into the 

 tissues after the manner of the parasite, I have not yet found it 

 possible to imitate nature, but in view of the overgrowths I have 

 obtained by a single slight stimulus it can no longer be doubted that 

 even in the absence of the bacteria the slow continual oozing into 

 growing tissues of the dilute acids, alkalies and other substances 

 named would produce a crown gall of any size desired. So long 

 as the stimulus is applied, and in nature it will be applied as long 

 as the bacteria are present in the tissues and continue to grow, so 

 long the growing tissues must respond. 



Before passing I wish once more to call attention to the italicized 

 names, and to urge all students of overgrowths to read Dr. Loeb's 

 book, since these tumor-producing substances, as I have said, are 

 those Dr. Loeb has found most active in starting the development 

 of animals out of unfertilized eggs. 



We will now pass to slides showing results obtained with 

 ammonia, dimethylamine, formaldehyde, acetic acid, and formic 

 acid (slides exhibited). 



III. The Mechanism of Overgrowths. 

 We now come to the inquiry embodied in the title of this paper 

 — what is the mechanism of these overgrowths? Is it a chemical 

 or physical action? It. is plain that the response is due to soluble 

 substances poured out, as a result of their metabolism, by parasites 

 present in the tissues, but given ofif in such small quantities that 

 they act not as a poison but as a growth-stimulus. That many 

 poisons when applied in minute doses do act as stimulants of one 

 kind or another is already well known, both in medicine and in 

 agriculture. That suspension colloids would be precipitated, pro- 



