A 
4 
: 
ig 
a 
. 
3 is 
ie Sg 
; 
i 
: 
a 
4 
: y 
1910] PENNINGTON—LONGITUDINAL COMPRESSION 279 
leaves and by tendrils. The researches of Bosr (3) have shown © 
that the protoplasm of other plants and plant parts is equally 
sensitive, although the plant cannot respond by a perceptible 
movement. In case growth is retarded, as it was in some of the 
experiments, the protoplasm must necessarily feel the effect. It 
has been demonstrated that there are interacting stresses in plants, 
of which a good example is afforded by twisted tree trunks. Force 
is required to cause such a twisting. If the weight of the top is the 
force, twisting must be accompanied by a shortening of the central 
cylinder or by an elongation of the outer layers. In either case, a 
longitudinal compression must be exerted upon the outer cells. 
It is not certain that the inner cylinder may become shorter, but 
it has been shown that wood cells may increase in length from the 
center to the circumference of a tree trunk. If this elongation of 
cells tends to lengthen the outer layers of wood in straight trunks, 
increased weight upon the trunk must alter the internal equilibrium 
of stresses, for it not only acts directly upon the outer layer, but it 
tends to increase the retarding action of the inner cylinder. More- 
over, it is not safe to say that the woody cylinder is so rigid that it 
cannot change, for we know that wood is more or less elastic, and 
that the limit of elasticity may be exceeded by either great pressure 
or great tension. If a bar of wood is forcibly bent and kept in 
that position for some time, a readjustment takes place so that 
the bar remains bent without the external force. The sensitive 
protoplasm in a cambium layer, which surrounds closely a cylinder 
of wood, must feel very slight changes that might occur in the 
cylinder. 
If the weight of the plant can furnish a stimulus for production 
of increased mechanical tissue, this stimulus must be lacking when 
the stem is relieved from supporting the weight of the plant. For 
the same reason an increased weight, equal to two, four, ten, or 
twenty times the weight of the plant, ought to produce a response 
by an increase in the strengthening tissue. Assuming that plants do 
respond to various stresses by becoming stronger, and that they 
remain weak if relieved of all stress, then the plants used in my 
experiments were placed under the most favorable conditions to 
Tespond to a given stress, for they were relieved of all stress except 
