374 RELATION TO ENVIRONMENT. 



IV. Annual Growth and Winter Protec- 

 tion of Shoots and Buds.* 



728. Winter conditions.! While herbs are 

 subjected only to the damp warm atmosphere 

 of summer, woody plants are also exposed dur- 

 ing the cold dry winter, and must protect them- 

 selves against such conditions. The air is dryer 

 in winter than in summer; while at the same 

 time root absorption is much retarded by the 

 cold soil. Then, too, the osmotic activity of 

 the dormant twig-cells being much reduced, the 

 water- raising forces are at a minimum. It is 

 easy to see, therefore, that a tree in winter is prac- 

 tically under desert conditions. Moreover, it has 

 been found by various investigators, contrary to 

 the general belief, that cold in freezing is only indi- 

 rectly the cause of death. The real cause is the 

 abstraction of water from the cell by the ice crys- 

 tals forming in the intercellular spaces. Death 

 ensues because the water content is reduced below 

 the danger-point for that particular cell. It was 

 formerly thought that on freezing, the cells in the 

 tissue were ruptured. This is not so. Ice almost 

 never forms within the cell, but in the spaces 

 between. Freezing then is really a drying proc- 

 ess, and dryness, not cold, causes death in winter. 

 To protect themselves in winter, trees provide 

 various waterproof coverings for the exposed sur- 

 faces and reduce the activity of the protoplasm 

 so that it will be less easily harmed by the los, o' 

 water abstracted by the freezing process. 



729. Protection of the twig. Woody plants 

 Fig. 427. protect the living cells within the twigs by the 



of Tw L y ie r -ch 1 estn W u i t g production of a dull or rough corky bark, or by a 



showing buds and . 



leaf -scars. (A twig 



with a terminal bud * This topic was prepared by Dr. K. M. Wiegand. 



Iec ted d or a this ) fi^ire?) t See discussion of Tropophytes in Chapter XLVII. 



