70 GROWTH AND WORK OF PLANTS 



110. The effect of freezing on plant tissue. Very few 

 plants are killed by actual cold or freezing of the tissues.* It 

 was once thought that in the freezing of plant tissue ice was 

 formed inside of the cells, and that the protoplasm was killed by 

 the cold. But this rarely occurs. The water freezes on the out- 

 side of the cell wall, and additional water flows out from the in- 

 side of the cell and continues to freeze there building up ice 

 crystals in the intercellular spaces (fig. 51). If the freezing con- 

 tinues long enough, so much water may be drawn from the proto- 

 plasm in the cell as to make it too dry and thus kill it. When 

 the buds freeze the ice crystals are formed in the intercellular 

 spaces. When the ice crystals thaw the protoplasm slowly ab- 

 sorbs the water into the cells again and is unharmed. Were it 

 not for the bud scales and other means for bud protection, the 

 water from the thawing ice crystals would evaporate and the 

 protoplasm would be killed. The effect of freezing on plant 

 tissue is, therefore, in most cases the same as that of excessive 

 dryness. 



111. Characters of winter buds and shoots. Winter buds 

 and shoots possess certain marks and other features which are 

 characteristic of the different kinds, so that a careful student of 

 these characters is enabled to tell the different kinds of trees 

 and shrubs from the winter condition of the shoots and buds.| 

 Some of these characters are as follows: The surface, whether 

 smooth or rough, shiny or dull, the color, the form of the lenti- 

 cels. The lenticels are minute elevations composed of corky 

 tissue with a minute opening, which serves the purpose of an inter- 

 change of air and other gases, between the tissues of the shoots 

 and the outside. The shape of the shoots is another character, 

 also the form and arrangement of the leaf scars, with their mark- 

 ings, the form and other characters of the buds, etc. The char- 

 acters of the following shoots will serve as illustrations. 



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

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



f See "A Key to the Genera of Woody Plants in Winter," 3rd edition 

 1908, by Wiegand and Foxworthy. Andrus & Church, Ithaca, N. Y. 



