CHAPTER NINETEEN 



THE MOVEMENT AND ACCUMULATION OF MATERIALS 

 IN STEMS 



Aside from growth the most important processes going on in 

 stems are those connected with the transfer of water, foods, and 

 other materials. The Hving cells of the stem secure energy for 

 chemical processes through respiration. They also assimilate 

 foods, and may temporarily, or permanently, accumulate food 

 and other substances. 



The lifting of water in stems. Nothing concerning the physiol- 

 ogy of plants has interested more people than the transport of 

 water from the soil to the topmost leaves of trees. Yet in spite 

 of much observation and experiment, the process is still only 

 partially explained. 



There can be no doubt that one of the principal factors in the 

 rise of sap is the evaporation from the cells of the mesophyll in 

 transpiration. The water thus lost is replaced by more water 

 passing into these cells from the adjoining water-conducting 

 tissue of the veins by osmosis. This is brought about by the 

 sugar and other substances in solution in these cells, as we learned 

 in Chapter XI. 



Water inclosed in tubes has a high cohesive power ; that is, 

 it holds together like a solid. If a pull is exerted on the upper end 

 of a column of water in the vessels of a tree, the column holds 

 together like a cord or wire, and the whole column is pulled 

 upward. As the water at the upper end of the water-conducting 

 tissue moves into the mesophyll cells, additional water is pulled 

 upward into the vessels of the blades, petioles, and stems. 



Transpiration is greatest and the largest amounts of water are 

 being lifted in trees during the summer. If at this season a hole 

 is bored into the trunk of a tree and an air-tight connection made 

 between this hole and a tube that has its lower end in a vessel of 

 water, the water is drawn into the stem, not forced out. This 

 indicates that there is more pull on the water from above than 



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