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 5 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 



