150 



A TEXTBOOK OF BOTANY 



[Ch. IV, 6 



any power which will draw water into the cells will lift the 

 water-columns in the ducts. This result follows from 



various chemical or 

 physical processes in 

 which water is ab- 

 sorbed ; and such se- 

 cretory actions are 

 believed to explain 

 the lifting of the sap 

 in the spring before 

 the leaves are de- 

 veloped. This ex- 

 planation is not yet 

 universally accepted, 

 many botanists still 

 holding that the liv- 

 ing cells along the 

 stem are the chief 

 factor in the process. 

 It must, of course, 

 be true that the 

 greater the height of 

 a tree, the greater 

 the difficulty of rais- 

 ing a sufficient 

 transpiration supply 

 against the increas- 

 ing hydrostatic re- 

 sistance, and the fric- 

 tion within the small 

 ducts. Thus a limit 

 is imposed to the 

 height of trees, which 

 potentially can grow 

 upwards indefinitely, but actually have heights approxi- 

 mately fixed for each kind. Our ordinary deciduous trees, 



Fig. 105. — Plan of the stem as a con- 

 ducting mechanism, arranged as in Figs. 11 

 and 166, with similar signs for protoplasm, 

 water, sugar, and proteins. On the left is the 

 pith, and then, in order, two ducts, a sieve- 

 tube, phloem parenchyma, cortex, and cork 

 with lenticels. 



