May 31,1924 Cell Sap Density and Environmental Conditions 
891 
SEASONAL CHANGES IN FOOD RESERVES 
The low osmotic concentrations of the conifers and the high concentrations 
found in the evergreen shrubs in the middle of the winter show a strong tendency 
toward a complete reversal of the results of the summer and autumn determina¬ 
tions. This was especially noticeable in the Taylor Canyon series. Douglas fir, 
white fir, alpine fir, and limber pine showed unusually low densities, while the 
two junipers were abnormally high. The evergreen shrubs, Artemisia tridentata , 
Cercocarpus ledifolius, Odostemon repens, Pachystima myrsinites and Ceanothus 
velutinus, showed strikingly high winter sap densities which increased with an 
increase in elevation. These results were confusing until micro-chemical tests 
for starch and oil were made on five conifers, Douglas fir, alpine fir, white fir, 
limber pine, and Rocky Mountain juniper, and the above five evergreen shrubs. 
The conifers gave a strong oil reaction indicating the presence of large amounts 
of oil and fatty substances in the leaves, while the evergreen shrubs gave only 
faint oil reactions with the common osmic-acid tests. 22 No substantial increase 
in the oil content of the evergreen shrubs was observed during the winter. 
Starch was not detected in any of the conifers or evergreen shrubs, in either 
January or February, when treated with an aqueous solution of potassium iodide 
and iodine (98, p. 28 ). With the rise of the sap in the spring strong starch reac¬ 
tions were obtained even without preliminary swelling of the starch grains with 
potassium hydroxid. These tests led to the conclusion that with the advent of 
cold weather in the autumn and early winter a large part of the starch in the 
conifers is converted into oil or fatty substances which are osmotically inactive 
and form emulsions having low osmotic concentrations. In the evergreen 
shrubs which showed little or no oil present the starch was evidently converted 
into soluble sugars, thereby materially increasing the osmotic concentration of 
the cell sap. 23 In the late winter or early spring a reconversion of starch and 
an apparent decrease in the amount of oil occurs throughout the tissues of the 
stem. This converted starch is apparently consumed in the formation of the 
spring growth, and it is not until summer that a fresh supply begins to be deposited. 
Starch, when found during the winter, was most abundant in regions remote 
from centers of conduction and in cells with thin or unlignified walls or large 
23 The definite increases noted in these observations on the conifers accord with those of Fischer (39), 
Preston and Phillips (W3), and Sinnott (117), who have listed the majority of the conifers of eastern 
North America among their “fat trees.” 
23 These conclusions are in general agreement with the findings of Fisher (89), LeClerc du Sablon (78), 
Niklewski (96), Preston and Phillips (103), Sinnott (117), Mer (93), Mitra (94), Miyake (95), Tuttle (118), 
(119), Coville (33), Petersen (100) and others who have discovered many important facts in regard to the 
character and seasonal changes of the food reserves in woody plants. Fisher (39), Niklewski (96), and 
LeClerc du Sablon (78) found that the amount of sugar varies during the year. Lidforss (81-83) found 
that with most of the wintergreen plants of South Sweden, during cold weather at least, the starch is almost 
entirely changed to sugar, although on the return of warm weather starch may be again deposited in the 
cells. Mitra (94) determined that the maximum starch content in apple stems and roots is reached in 
October and November and that the total sugars increased in January and March. Rigg (107) also has 
observed the accumulation of sugar at the expense of starch, during the winter, in the evergreen angiosperms 
of the Puget Sound Region. It is important to note that Coville (23) found that the exposure of plants to 
cold results in the transformation of stored starch to sugar, with the consequent development of high osmotic 
pressures. He advances the theory that, during the process of chilling, the starch grains stored in the 
cells of the plant are at first separated by the living active cell membranes from the enzyme that would 
convert the starch into sugar, but that when the plant is chilled the vital activity of the cell membrane is 
so weakened that the enzyme permeates it, comes in contact with the starch and transforms it 
into sugar Sinnott (117) suggests that the ease with which water or substances soluble in water have access 
to the plant cell is probably a determining factor in the extent to which starch is changed to sugar, and that 
differences in the type of food reserve may be due to differences in water content of the various storage 
cells, resulting in enzyme activity, or differences in the ease with which enzymes have effective access to 
the storage cells. 
