STONE: CONTACT STIMULATION 471 
and well developed on those in contact with fine excelsior. The 
number of secondary roots or those in contact with the fine excelsior 
averaged 18 to a plant and the ratio of the total length of the entire 
root system of the normal and fine excelsior was I to 6 in favor of 
the latter. Measurements made of the total surface area of a single 
typical root from one of the normals and one of the fine excelsior showed 
that the total surface area of the latter was over three times that of 
the normals. These experiments show, at least in young seedlings, 
that roots respond to contact and that the response is confined very 
largely to these organs, although more than one type of growth 
correlation may occur. They indicate also that different species will 
respond to contact in a different manner. In other words, secondary 
root development is stimulated more in some species than in others 
by contact, and this excessive development of the secondary root 
system is correlated with a lesser development of the primary root 
system. 
Soil Particles 
Since roots are sensitive to contact to various materials it would 
naturally be supposed that the nature of the soil constituents or 
particles would exert an influence upon growth and configuration of 
plants, and particularly upon the root itself. As contact is effected 
by the surface area involved, variation in the size and shape of the 
soil particles would be expected to produce different results. Conse- 
quently, a series of experiments were carried on, but not completed, 
with the idea of determining what effect, if any, soil particles have 
on the growth of roots, and how the various-sized particles effect 
development. For this purpose we had at our disposal several care- 
fully prepared grades of gravel, sand, silt and clay which had been 
sifted through sieves and bolting cloth. The size of the particles 
ranged from 16mm. to0.I mm.and in some cases to.05 mm. The ex- 
periments were conducted in glass jars filled with water previously boiled 
to exclude air. Each jar was completely filled with some particular 
grade of material which had previously been thoroughly washed with 
water and sterilized. We thus had a medium in which the particles 
of gravel, sand, etc., were surrounded by water, and as far as possible 
free from air. A fine-mesh cotton netting was placed over the jars 
on which rested the seeds, and as germination took place the radicles 
penetrated downward between the submerged soil particles. All 
experiments made with soil particles in water were carried on in 
darkness in a moist chamber where transpiration was limited and 
the temperature condition alike. The plants were in fact under 
identical conditions throughout, except as regards the substratum. 
