June, 1923 ] 
MURNEEK — CHANGES IN BARTLETT PEARS 
321 
It should be pointed out here that, when potassium iodid is used as an in¬ 
dicator of the amount of starch present, the sections will invariably stain 
solid blue as a result of the intensity of the color and the translucency of the 
cell walls, although only a small percentage of the volume (5 percent or 
less) is occupied by starch (text fig. 3, a). 
Discussion of Results 
From the foregoing data it may be concluded that a series of morpho¬ 
logical and histological changes of the dermal and cortical regions of the 
pear may be largely responsible for the marked physical changes of these 
tissues in respect to resistance to pressure. Naturally the cells of the cortex 
are of the greatest importance here, since by far the predominating per¬ 
centage of the tissues involved were those of the cortical region. While the 
average seasonal decrease in resistance to a certain physical pressure (14) 
amounted to approximately 30 percent, the cortical cells had increased in 
size during this period by 94 percent. At the same time, the walls of these 
cells showed a reduction of 14.8 percent in thickness. Consequently an 
assumption may be made of a probable correlation between the physical and 
the morphological changes in the cortex. One can only conjecture, however, 
as to the exact numerical relations here. So, too, it is possible that, as a 
result of a gradual hydrolysis of starch into sugars, a greater turgidity is 
attained by the cells of the cortex during the latter part of the season, re¬ 
sulting in a lowering of physical resistance. Such a mechanical explanation 
is offered by Hawkins and Sando (4) for the greater resistance to wounding 
of the epidermis of cherries and of various other small fruits. In the cases 
of these fruits, however, changes in turgidity, caused by lowering of tempera¬ 
ture, are assumed to be, due to probable differences in the coefficient of 
expansion of the cell walls and the cell contents. The point is very sugges¬ 
tive, since it has been shown by Lewis, Murneek, and Cate (10) that a 
marked increase in resistance to wounding of Bartlett pears was obtained 
when the fruit was kept for 24 hours at room temperature (summer). 
Naturally there was a considerable loss of moisture during this time, with a 
consequent profound effect on the turgidity of the cells. 
Undoubtedly the chemical alterations of the various constituents of the 
cell walls should be considered here as an additional factor. It is a well 
known fact that the hemicelluloses, pentosans, and particularly pectic 
compounds, undergo’radical changes at the time of maturity of the fruit. 
By careful chemical analyses, Magness (11), for instance, has shown that 
as the pear ripens on the tree there is a conspicuous decrease of alcohol- 
insoluble, acid-hydrolyzable substances other than starch. In many 
instances the decrease in these polysaccharides amounted to 50 percent of 
the total present, thus indicating that as the fruit develops much of this 
material, other than starch, is hydrolyzed. Magness points to the reduction 
of pectose and related material, which is thought to be largely responsible 
