1907] HIBBARD—FORMATION OF MECHANICAL TISSUE 375 
Helianthus annuus 
Twenty-six sunflowers were subjected to compression by the 
different methods explained above, the method depending upon the 
size and age of the plant used. Seedlings were cultivated in 20°™ 
pots in the greenhouse. The duration of the experiments varied from 
two to five weeks. The weights varied from 400 to 21008. Asa 
general result of this experiment one may say that a compression 
strain will call forth slightly increased mechanical development in a 
majority of the plants used. This expresses itself in a slightly greater 
thickness of cell wall and xylem cylinder. The phloem bundles are 
also slightly larger. 
Fuchsia speciosa 
Five small Fuchsia stems were subjected to a compression strain of 
185%". No bending was present to modify the anatomical structures. 
Sections were taken from the stems within the region of compression 
at distances from the ground varying from 5 to 8. 5°™. Sections of 
the normal stems taken at corresponding places were selected for 
comparison. With the aid of an eye-piece micrometer, measure- 
ments of the xylem cylinders were made in two directions at right 
angles to each other. The average of these two measurements was 
taken as the average width of the xylem ring. Data so obtained 
indicated a greater thickness of xylem in the tension plant than in 
the control, but the increased mechanical development was not 
strong. 
Vinca major 
Eight young upright Vinca stems were subjected to compression 
by hanging the weights upon them. . To prevent bending, the plants 
were tied to a support in the usual way, the final weight carried 
by each plant being 200%". The period of experimentation extended 
from March to to March 2 5. Control plants were selected with 
teference to the same age, growth, and vigor. Sections taken in the 
Tegion of stress were compared with sections taken at corresponding 
Places in normal plants. 
Observations under the microscope indicated, in a majority of the 
plants experimented upon, that a compression strain causes slightly 
increased development of mechanical tissue. 
