484 Forestry Quarterly 



for this precipitation by the greater concentration of the salts in 

 the sap as water is removed to form ice crystals. 



Another investigator, working with wintergreen plants of 

 Southern Sweden, has found that with most of them, at least 

 during cold weather, the starch is almost entirely changed to 

 sugar, though on the return of warm weather starch may again be 

 deposited in the cells. He assumes that this sugar is formed during 

 cold weather as a means of protecting the plant against freezing by 

 lowering the freezing point of the sap due to its increased concen- 

 tration. 



The author performed a long series of experiments to test these 

 theories, using the leaves, twigs, flowers and fruits of fruit trees in 

 various stages of development besides the leaves from a large 

 number of cultivated herbaceous plants, and he found that the 

 killing temperature of plant tissues that kills at relatively high 

 temperature was reduced whenever the sap density of the tissue 

 was increased. With one or two possible exceptions, there was no 

 indication of the precipitation of the proteids in the cell sap at the 

 killing temperatures. 



Other interesting conclusions may be gleaned from the author's 

 summary. For example, with a few exceptions, there is no indica- 

 tion that the rate of thawing has anything to do with the amount 

 of killing at a given temperature. There seems to be no constant 

 relation between the rate of growth of plant tissue and resistance 

 to low temperature. Young leaves of fruit trees kill at higher 

 temperature than do old, mature leaves, while young leaves of 

 lettuce withstand a lower temperature than do the older leaves. 

 The most important feature affecting the hardiness of plant tissue 

 is maturity, that is, the condition of resistance that the plants 

 reach during the winter dormant period. Maturity in case of the 

 cambium may be intimately associated with the process of drying 

 out. However, this cannot be true in case of the cortex of winter 

 twigs. There is very little difference between the moisture con- 

 tent of unfrozen cortex in seasons when it is very tender and in 

 seasons when it is hardy. The wood at the base of the trunk and 

 at the crotches of all rapidly growing branches seems to reach a 

 condition of maturity more slowly than does most other tissue. 



C. D. H. 



