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 i8 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 i6 mm. to o.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. 



