3 H 
AMERICAN JOURNAL OF BOTANY 
[Vol. IQ, 
naturally arose as to what may be the underlying causes of the gradual 
change of tissues of the epidermis and the cortex to resistance to pressure of 
the character above noted. One is led to assume that the factors involved 
Fig. i. Graphs showing records of pressure tests on Bartlett pears and length of 
shipping seasons. Rogue River Valley, Oregon, 1918-1919, and Willamette Valley, Oregon, 
1920-1921. 
are primarily physical, but in accordance with their physical alterations cells 
undergo considerable chemical changes. No chemical investigations, how¬ 
ever, were attempted at this time. The following is a report on studies of 
morphological and histological modifications of the Bartlett pear as revealed 
during the comparatively brief period of harvesting and shipping of the 
fruit. 
Morphological and Histological Investigations 
Material and Methods 
A concise account may be given here of the nature and extent of injury 
caused by the physical test, i.e., by the tip of the plunger of the pressure 
apparatus. The tissues affected form a conical area, with the apex toward the 
axis of the fruit (PI. XXIV, figs. 4, 5). The epidermis, subepidermis, and a 
considerable number of immediately underlying cells of the cortex are not 
ruptured, excepting in extremely ripe pears, but are torn in a circle, forming 
a plug which crushes against the cells of the cortex. A decided line of 
