STONE: CONTACT STIMULATION 473 



much difference in the growth of hypocotyls and roots in the different 

 grades. This feature is associated with the greatly accelerated 

 development of the secondary root system, and extending from the 

 coarser grades to the finer ones. Other than the production of 

 secondary roots in the plants in the i6-8 mm. grades there was little 

 difference between the growth of the latter and those of the normal. 

 There were no secondary roots in the normal grades or water cul- 

 ture plants in this case, although they were fairly well established in 

 the 1 6-8 mm. grade, from which grade the increase in numbers and 

 total length of the secondary roots were quite noticeable. The 

 average development of the primary and secondary organs as well as 

 the surface area of the same was greater in the contact plants than in 

 the normals. Soil particles and excelsior submerged in water have a 

 similar stimulating effect on mustard as will be seen by comparing 

 Tables i8 and 19. 



Theoretical Considerations 



While the general tendency of plants and plant organs is to avoid 

 contact with one another, the histological units or cells which com- 

 posed the individual are in contact with one another, and the same 

 holds true to a certain extent with different organs when in the em- 

 bryonic or bud stage. It is the exception rather than the rule to find 

 the various members or organs of different plants, or even those of 

 the same plant such as roots, branches, leaves, etc., in contact with 

 one another, or in other words it appears to be a universal law in 

 nature that the various organs of plants occupy space by themselves. 

 Uniformity and regularity in the arrangement of cells and organs is 

 more common to the lower than to higher organisms, since in the 

 higher organisms this feature is sacrificed to some extent by biological 

 necessity and adaptation. Primarily the arrangements of organs in 

 plants or angles of divergence are determined by laws which are 

 common to gravitational and electro-magnetic phenomena, and the 

 arrangement of the various organs of plants appears to be determined 

 by the action of these forces upon their ultimate structural units, 

 molecules, micellae, atoms, electrons, or whatever they may be. 

 The angles which various organs assume in plants closely resemble 

 those which are illustrated in the formation of certain types of crystals, 

 and the behavior of iron filings under the influence of a magnet. 

 Plants are susceptible to all of the common environmental influences 

 which surround them, but the modus operandi of these various external 

 agencies on protoplasm is little known and especially concerning the 

 mechanism and nature of conductivity of impulses. The reaction to 

 contact results from a mechanical impulse, inasmuch as when the 



