STONE: CONTACT STIMULATION 479 



acceleration, but these apparently affect the outer tissues, and to a 

 certain extent the underlying vital layer. Dr. H. von Schrenk (7) 

 and Dr. E. F. Smith (6) have shown that ammonia compounds cause 

 intumescences in cauliflower. The absorption of the chemical sub- 

 stances by the roots in some cases produces a rupturing of the epidermal 

 tissue which is followed by cork formation, and in cases of malnutri- 

 tion excrescences are often formed on the fruit. Finally, contact 

 stimulation has an important bearing on experimental work, especially 

 with that done with plant food, fertilizers or soils, inasmuch as stakes 

 and wires are often employed as support. Any differences existing in 

 the contact of the plants, either by the use of supports or that occa- 

 sioned by the proximity of plants to one another, thus causing contact, 

 would be effective in modifying the results. The relative height and 

 development of two plants which would affect the contact surface 

 would also be important factors in experiments of this nature. Indeed 

 in this respect we have noticed on more than one occasion experiments 

 where differences existing in the degree of contact would account for 

 all the variations in the growth of the plants that were supposed to 

 be due to other causes rather than to the particular treatment which 

 they received. 



LITERATURE CITED 



1. Hall, A. D., Brenchley, W. E., and Underwood, S. M. The Soil Solution and the 



Mineral Constitutions of the Soil. The Jour, of Agr. Sci. 6: 278-301. Sept. 

 1914. 



2. True, R. H., and Oglevee, C. S. The Effects of the Presence of Insoluble Sub- 



stances on the Toxic Action of Poisons. Bot. Gaz. 39: 1-21. 1905. 



3. Stark, Peter, Ber. d. d. Bot. Ges. 33^: 389. 



4. Stone, George E. The Power of Growth Exhibited by Ferns. Bull. Torrey 



Bot. Club 36: 221-225. May 1909. 



5. Pfeffer, W. Druck und Arbeitsleistung durch Wachsendepflanzen, p. 93. 



6. Smith, E. F. Mechanism of Tumor Growth in Crowngall. Jour, of Agr. Res. 8; 



Jan. 1917. 



7. Schrenk, H. von. Intumescences Found as a Result of Chemical Stimulation. 



Ann. Rept. Mo. Bot. Gard. 16: 125-148. 1905. 



