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. 
I9I4. 
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: I-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. 
