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TEXTBOOK OF BOTANY 



is conditioned not only by the effects of root pressure but also by the 

 presence of gases in the xylem tissues. The flow is greatest during the 

 period when freezing temperatures occur at night and davtime tempera- 



Fig. 136. Daily variations in the diameter of the trunk of a Monterey pine. The 

 contraction of the wood cells is a result of tension on the water columns due to 

 transpiration. Stems of plants shrink in diameter in the afternoons when the rate 

 of transpiration is relatively high. Data of D. T. MacDougal (1936). 



tures are well above the freezing point. At this season the sugar content of 

 the tree is high. Transpiration is low because the buds have not yet 



\ ■* I ^ u / 



Fig. 137. Method of collecting "sap" from sugar maple. 



opened and absorption of water by the roots is active. When the twigs 

 and smaller branches are warmed by the sunlight, the gases in the xylem 

 expand. The resulting pressure added to the pressure developed in the 

 roots causes the sap to flow from the tap-holes in the trunk (Fig. 137). 



