898 
Journal of Agricultural Research 
Vol. XXYII1, NO. 9 
tests were made during a period of unusually wet weather, which probably accounts 
for the small differential between the sap of the parasite and that of the host. 
Again, osmotic pressure has been found to be one of the fundamental variables 
among the forces by which solutes are drawn from the tissues of the host, a 
discovery that has contributed to the knowledge of the physiology of parasitism. 
In the preceding pages the great importance of knowing the reactions between 
the environmental factors and the various physiological functions of the plant 
have been repeatedly emphasized. The variable results noted in the winter sap 
density studies could not have been properly interpreted without an examina¬ 
tion of the starch content of the leaves. It has been shown that, through the 
osmotic concentration of the sap, the initial changes preliminary to early spring- 
growth may be detected by the rise and consequent effect on the density of the 
sap in the leaves of evergreens. The seasonal changes in carbohydrate conver¬ 
sion may also be followed by making use of the osmotic pressure method. Of 
no small significance is the applicability of the method to the study of the water 
balance of the plant—changes in its water relations due to moisture supply and 
water loss. 28 
The fact has been repeatedly emphasized during this investigation that no 
one factor or group of factors should be singled out, to the exclusion of all others, 
as an explanation of all physiological phenomena, and furthermore, that the 
normal physiological activities of the plant can only be interpreted when they 
are properly correlated with both the climatic and soil conditions of the environ¬ 
ment. 
RELATION OF DENSITY OF SAP TO HARDINESS 
The intimate relation of cell sap concentration to drought resistance has 
already been brought out. There is also an apparent relation to winter hardi¬ 
ness. This has been indicated by a number of investigators who have correlated 
the transformation of starch into sugar and the resultant increase in the density 
of the cell sap in the winter with frost resistance. Lidforss (81, 82) and Petersen 
(100) have shown that a close relation exists between the sugar contents and 
frost hardiness; the more sugar the less injury from frost. The sugar is believed 
to protect the living protoplasm against frost injury by lowering the freezing 
point of the cell contents. 29 
Numerous theories 30 have been advanced concerning the mechanias of frost 
injury, such as failure of the protoplasm to regain water lost when ice crystals 
form in the tissues, the precipitation of proteins, or other metabolic changes 
accompanying low temperature. A number of empirical observations in the 
Intermountain Region indicate that the injurious effects come when ice crystals 
form. Wet shoots freeze at a higher temperature than dry shoots. Wet leaves 
show more serious injury at a given temperature than dry leaves. A period of 
cold weather immediately following a rain or wet snow is thus more dangerous 
than a period of cold drj 7 weather. With the approach of spring there occurs an 
increase in the tenderness of buds. 31 
28 Bates and Zon (5) have pointed out that the termination of the season's photosynthetic activity may 
possibly be determined by the same method. 
29 Fischer (89), Le Clerc du Sablon (78), and Niklewski (96) have also found that, while the amount of 
sugar in the stem varies during the year, the maximum of sugar in winter makes for greater frost hardiness. 
30 A number of these theories are reviewed and a bibliography of 50 citations dealing with the hardening 
process in plants is given by Harvey (62). 
31 These observations are in accord with the experimental results obtained by Harvey (62, 63), Chandler 
(17, 18), Rosa (109), Hauch and Ravn (64), Harris and Popenoe (51), and Ohlweiler (97). On the other 
hand, Salmon and Fleming (111) maintain that turgidity of the tissue as influenced by physiological 
drought appears to have more influence than sap density on the ability of a plant to resist winter killing. 
