412 



Plant Physiology 



force which holds water in the tissue. This, in turn, de- 

 pends upon the relative proportion of water to cell-wall 

 and protoplasm." Measurements were made by Wiegand 

 of seven species of trees frozen at a temperature of 18 

 C. and of seven species which failed to freeze. The com- 

 parative data for two species in each group are presented 

 by the following table : 



LABORATORY WORK 



Effect of heat and cold upon germination. Expose selected 

 dry seed of barley or peas for 1 hour to dry-oven temperatures 

 of 50 C., 75 C., and 100 C. Place 25 of each lot in a ger- 

 minator together with an equal number of control seed and 

 determine the effect upon the percentage of germination. In 

 the same way employ seeds of the above plants in water at the 

 temperatures above given, and test similarly. For further 

 comparison it is also desirable to employ in both experiments 

 a lot of seed which are just beginning to germinate. Discuss 

 the results. 



