280 BOTANICAL GAZETTE [OCTOBER 
the downward pull of the weights. In no case, however, were they 
found to become stronger mechanically. It must be concluded that 
a longitudinal compression, acting by itself and acting continuously 
upon a plant stem, cannot stimulate it to greater activity, nor to 
an increased strengthening of the tissues that are already present. 
COMPRESSION IN RELATION TO OTHER STIMULI 
For convenience, we may separate the stimulatory influences into 
three groups: the so-called internal stimuli, environmental stimuli, 
and strains or stresses. These internal stimuli may be due indirectly 
to environmental conditions. Since we know little or nothing 
about them, it is convenient to refer to this group all those phenom- 
ena which we cannot attribute to definite external conditions or 
stimuli. Environmental stimuli include gravity, light, heat, 
moisture, etc. Strains or stresses are tension and compression. 
In nature the various influences are intimately associated with 
each other, and often so modify one another that observers and 
sometimes experimenters have overlooked certain influences and 
thus given undue weight to others. As an example we may speak 
of Hartic’s observations and CresLAR’s experiments in which 
‘pressure only was considered, where evidently gravity, tension, and 
perhaps other influences may have played an important par t. 
Others, who were not able to account for observed phenomena 
by certain external stimuli, explained them by saying that they 
are due to internal stimuli. As an illustration, a quotation pom 
Massart (19) will serve very well. In speaking of root swellings 
and eccentric growth in Ficus roots he says: 
L’épaississement asymétrique est régi par la gravitation, par la lumitre et 
par des excitants internes, encore indeterminés. 
Experimenters who examine more closely into the reactions 
of plants are forced more and more to the conclusion that the 
plant acts as a unit in a self-regulatory manner to bring about th x 
greatest good to itself. Nrwcompe (21) was the first to point 
out the necessity of considering the self-regulatory power of plants 
with special reference to their response to stress.. UrsPRUNG 
(32, 33) came to practically the same conclusion after a sete of 
observations upon eccentric growth in trunks and branches. 
