The Temperature Relation 411 



are complex effects, the permeability of the protoplasm 

 being also affected. 



250. Buds. The relation of buds to cold has received 

 careful attention by Wiegand. He finds that ice may form in 

 a large number of species when the temperature falls as low 



FIG. 111. Section of a bud of Populus nigra frozen at o F. sectioned and 

 photographed in the open ; light areas are ice crystals. [After 

 Wiegand.] 



as 18 C. At this temperature it may be formed in large 

 quantities and is more abundant in cortical and paren- 

 chymatous tissues than in meristem. When absent at this 

 temperature, it may be assumed that the tissue is made up 

 of very small cells with thick walls and low water-content. 

 This is explained by the fact that " the degree of cold nec- 

 essary to cause the separation of ice is proportional to the 



