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 16-8 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 16-8 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 18 and Ig. 
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 
