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 efi'ect 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 



