GROWTH AND WORK OF PLANTS 



or by hairs alone, as is the case in some buds, is generally supposed 

 to be a protection against freezing. This is not strictly true, for 

 ice is abundantly formed within the buds during very cold weather 

 even in buds well covered with scales or hairs. It is rather a pro- 

 tection against the effects of freezing, or more properly speaking, 

 it is a protection against the loss of water from the delicate tissues 

 of the bud. This protection applies then to buds in climates 

 where the resting season for vegetation is dry and hot as well as 



in climates where 

 the resting season is 

 very cold. When 

 freezing takes place 

 (in plant tissues a 

 little below the 

 freezing point for 

 water outside) the 

 ice is rarely formed 

 inside of the cells 

 in the protoplasm. 

 The ice forms on 

 the outside of the 

 cells in the inter- 

 cellular spaces. As 

 freezing continues 

 water is drawn from the cells and added to the ice crystals in the 

 intercellular * spaces. The effect of freezing then is to draw water 

 from the cells, i.e., it is a drying effect. If the buds had no protec- 

 tion on the outside, the ice would gradually vaporize and escape. 

 The bud coverings, however, prevent excessive loss of water, and 

 when warmer weather comes the ice crystals in the intercellular 

 spaces melt and the water is drawn again into the protoplasm of 

 the cells by osmosis (paragraph 352). Bud coverings probably 

 protect the young growing plant from mechanical injury also. 

 For the study of buds see Chapter VIII. 



* See " Some Studies Regarding the Biology of Buds and Twigs in 

 Winter," Bot. Gaz., 41, 373-423, 1906. 



Fig. 48. 



Section of frozen bud of Populus dilitata showing ice from 

 water drawn from the bud leaves (from K. M. Wiegand). 

 The white crescents are masses of ice between the bud scales. 



