138 Bulletin 79 



The similarity of the curves of freezing in the dead tissues of 

 the cacti is due purely to a physical condition, while the differences 

 in the curves of the living tissues is due t^ a biophysical condition. 

 The absence of living protoplasm in the cells appears to be respon- 

 sible for the differences observed in the curves. 



EFFECTS OF FREEZING THE TISSUES 



Taking up the third problem, it is well known that in freezing 

 plants ice generally appears first in the intercellular spaces of the tis- 

 sues. Since these intercellular spaces normally contain air and not 

 water, the presence of crystals of ice in them suggests that previous to 

 freezing the water must have been secreted from neighboring cells 

 with the lowering of the temperature. This low temperature 

 causes the water to separate from the .cell solutions and freeze. 

 This leaves the remaining cell-sap solution stronger in soluble 

 substances, and consequently a lower temperature is required to 

 cause more of the water to separate and freeze. It is easily under- 

 stood that the cell contains a stronger sap solution after a part of the 

 water has frozen. In some plants this solution may become strong 

 enough to kill the protoplasm of the cells affected. 



The small amount of material which is dissolved in the storage 

 water in cacti is so unimportant that it can do little or no harm to 

 the protoplasm of the cells, even when relatively low temperatures 

 have caused a large amount of the water of the cell-sap solution 

 to be secreted and frozen. 



With certain precautions the behavior of the frozen plant tissues 

 may be studied easily under the microscope. A Ganong tempera- 

 ture stage which serves as well for studying the behavior of the 

 protoplasm under low as under high temperatures was used. F'or 

 this study the stage was fixed in the usual way to the microscope, 

 and the triangular copper basin was filled with a mixture of cru.shed 

 ice and salt. To lower the temperature further, a vessel was filled 

 with the same freezing mixture, in which the triangular copper 

 basin of the temperature stage was partly embedded. In this way 

 the metallic part of the temperature stage, which is protected by 

 a thick felt covering to prevent radiation, and which is located 

 directly above the stage of the microscope, could be kept easily at 

 a temperature of — 5° C. (23° F.). The temperature, however, 

 could not be reduced below this point. The holes in the sides of 

 the temperature stage where the glass slide is inserted were filled 

 with cotton batting to assist in maintaining a uniformly low tern- 



