354 



PLANT PHYSIOLOGY 



frost. Such hardening is always accompanied by an increase 

 of the sugar content in the cells of the plant. A similar accumu- 

 lation of sugar may be observed in potatoes stored at a tempera- 

 ture about 0°C.; the increase of sugars is readily perceptible to 

 the taste. This unpleasant flavor is easily removed by keeping 

 such sweetish potatoes for some time in a warm room. 



Net fr^xen 





Fig. 113. — Protective influence of sugar in freezing expressed plant juice {after 



Newton) . 



Hardening, however, is not entirely due to the accumulation 

 of sugar; the protein substances of the protoplasm also undergo 

 changes, as a result of which they obtain the capacity of being 

 coagulated irreversibly only at temperatures lower than before 

 hardening. Especially marked seasonal variations in hardiness 

 may be observed in trees. The needles of conifers, for instance, 

 are able to withstand winter temperatures of —30 to — 40'^C., 

 while in summer they are killed at — 8°C. 



