322 
AMERICAN JOURNAL OF BOTANY 
[Vol. io, 
for the thickening and cementing together of the cell walls, as a possible 
cause of the lowering of resistance to pressure of the pear. 
There is a prevalent opinion that as the fruit ripens a gradual dissolving 
of the middle lamellae takes place (18, and others). Sections through the 
wounded areas of even comparatively ripe pears showed, however, that the 
cells had very rarely separated along the middle lamellae, but that the 
characteristic tearing was across the cell walls. This fact, of course, does 
not diminish the importance of the possible changes of pectic substances in 
the middle lamellae and the cell walls proper. Hornby (5), for instance, 
found an increased pectin content in those parts of the potato and other 
tubers that were most exposed to light. The writer’s own experience in¬ 
dicates that the pigmented part of the pear, the side of the fruit most ex¬ 
posed to light, has a greater resistance to wounding than the green portion. 
Of minor consequence here is the reduction of the starch content from 
5.5 percent to approximately I percent of the total volume of the cell. This, 
too, may play a part in the relative resistance to physical changes of these 
cells. Again, because of expansion of the cortex and subepidermis, greater 
separation of the groups of stone cells may be effected, thus lowering to 
some extent the combined resistance to pressure of the tissues as a whole. 
Apart from the cortex, the seasonal differences in size and distribution 
of the epidermal cells are of interest. Cells of the subepidermis show a 
relative increase of approximately 65 percent in size. Close to the outside 
this increase of course was much lower, amounting to 40 and 14 percent 
respectively for the first layer of subepidermis and epidermis. A decrease 
in thickness of the cuticle of 19 percent may also be noted. Undoubtedly 
all these factors contribute to the measurable physical changes of the 
tissues of this region. So, too, one may expect changes in the cell walls here 
quite similar to those suggested for the cortex proper. 
In conclusion, it may be said that the lowering of resistance to physical 
pressure of the cortex and epidermis of the Bartlett pear may be due in a 
large measure to the morphological and histological changes of the tissues 
involved. In order of decreasing importance the factors under considera¬ 
tion could possibly be arranged as follows: 
a . Cortex: Increase in size of cells and decrease in thickness of cell walls. 
b. Subepidermis: Increase in size of cells and possible decrease in thick¬ 
ness of cell walls. 
c. Epidermis: Increase in size of cells and decrease in thickness of 
cuticle. 
d. Decrease in amount of solids (starch) in the cells. 
e. A wider separation of the groups of stone cells in both subepidermis 
and cortex. 
Summary 
1. Changes in maturity of the pear fruit have been commonly expressed 
in terms of chemical differences of the cell contents. 
