478 BROOKLYN BOTANIC GARDEN MEMOIRS 
When roots are growing between large boulders and are restricted 
in their development, the flow of tissue is such that the root may 
become enormously flattened. The reaction in such cases is similar 
to that produced by a wound, although evidence of wounding may be 
entirely absent. Such roots, however, often lift enormous weights 
which would indicate that considerable osmotic tension exists in the 
cells. When rapidly developing organs, such as a squash, for example, 
is placed in a harness and subject to weight, it will assume a much 
distorted shape which illustrates stimulated growth phenomena. 
Again, when ferns and even delicate mushrooms push through con- 
crete they show little or no evidence of wounding, although the reac- 
tions in such cases are characterized by a large increase in the osmotic 
tension of the cell, sometimes equaling 50 atmospheres (4). The 
experiments of Pfeffer (5), in enclosing roots in plaster casts, thus 
restricting growth and greatly increasing the osmotic tension of the 
cells, are typical of this class of responses. To what extent, if any, 
cell fusion of sexual elements, development of attachment organs in 
fungi and algae, and outgrowths in spirogyra filaments when in con- 
tact with certain crystals, are related to contact, is problematical since 
in some instances chemotropic phenomena would have to be carefully 
differentiated from any other which might prevail in interpreting such 
phenomena. Also to what extent contact stimulation may influence 
cell enlargement and cell division resulting from the intrusion of 
foreign elements would be merely a matter of speculation at the 
present time. There are, however, numerous instances of cell enlarge- 
ment associated with crystals (raphides), pathogenic and non-patho- 
genic organisms, which resemble contact stimulation, although the 
recent important contributions on this subject by Dr. E. F. Smith (6) 
would indicate, in some cases at least, that cell responses to pathogenic 
organisms are associated with chemical or physical phenomena. Some 
of the responses of plants associated with pathological phenomena are 
not characterized by cell stimulation, but with color reactions due 
apparently to excretions from organisms. Intense color reactions 
are also associated with marked cell proliferation caused by chemical 
substances absorbed by plants from the soil, as shown by the reactions 
of Platanus Orientalis to the toxic properties contained in illuminating 
gas. These reactions, however, as are similar ones in poplars and wil- 
lows, which develop large masses of parenchyma under the bark and 
cause rupture of the same, are associated with two factors, namely, 
the direct effect of the toxic substances on the cambium inducing 
rapid cell division, and decrease in the tissue tensions of the cortex 
following the collapse of the same by poisons. Various chemical 
substances (banding substances) applied to trees produce local growth 
