SPECIFIC GROWTH FACTORS 331 



open. The closed stomata are more acid, and this may be associ- 

 ated with the ease of heat coagulation (Weber, 1926). 



The effect of cold seems to be somewhat different from that 

 produced by heat. As the plant cools down to near the freezing 

 point (0° C), the water which is in the cell walls and the inter- 

 cellular spaces freezes first because of its lower osmotic pressure. 

 In this way water is withdrawn from the cell and the freezing 

 point of the cells becomes lower and lower. It has been estimated 

 that ice crystals will not form in the cells themselves until a 

 temperature of —20° C. is reached. It is for this reason that the 

 degree of injury by cold weather varies with the water content of 

 the cells; juicy, succulent cells are much more easily injured by a 

 drop in temperature than cells with a low water content. A part 

 of the injury is mechanical, caused by the lacerating of the tissue 

 by the ice crystals whether within or without the cells. It may 

 also happen that certain harmful physical changes, similar to the 

 coagulation produced by heat, may result. As the osmotic con- 

 centration of the cell sap becomes progressively higher, the time 

 may come when the proteins are " salted out" or precipitated by 

 the excessively high salt content of the remaining liquid. 



Whatever the injurious effect of freezing may be, it is certain 

 that it varies with the amount of water present and that the 

 hardiness problem is very closely connected with the water supply. 

 Illustrations of this point have been multiplied the past few years 

 to the place where there can be no further doubt about it : tissues 

 with a high water content and a low concentration of osmotically 

 active substances are much less hardy and resistant to cold weather 

 than tissues with a low water content and a high concentration of 

 sugars, salts, etc. Wheat grains frozen when their water content 

 was 69.4% showed only 1% germination, while those frozen when 

 the water content was only 45% showed 84% germination (Whit- 

 comb and Sharp, 1925). The specific gravity of unfrozen Citrus 

 fruits is higher than that of frozen ones after they have been 

 subjected to freezing temperatures, and those leaves that have a 

 lower osmotic concentration in their cell sap freeze at a higher 

 temperature (Haas, 1928). Peach bud hardiness varies in different 

 varieties inversely with the amount of water per unit of dry mat- 

 ter (Johnston, 1923). In the cells of Picea canadensis leaves there 

 is a rapid fall in osmotic pressure from over 20 atmospheres to 

 16-17 atmospheres as the cells pass from the winter to the summer 



