STEMS 243 



and all of the older ones have been disproved, so that we are 

 still waiting for an explanation that will stand the test of 

 experiment. 



That the path of ascent ijrthrough the vessels of the xylem, 

 and not through cortex or phloem or pith, may be demon- 

 strated by a simple experiment. A stem of corn or sun- 

 flower or balsam is cut off and placed in water for an hour. 

 Then it is transferred to a vessel containing water stained 

 with cheap red ink (a solution of eosin), and exposed to 

 diffuse light. A few hours later, sections of the stem will 

 show the xylem vessels stained red, the ascending water 

 having stained its path. Of course the stain may spread 

 somewhat into adjacent cells. It must be remembered 

 that the xylem vessels are not living cells when they become 

 the best water-carriers, so that the movement of water 

 through them is not like the movement of water from one 

 living cell to another. When the great distance to be trav- 

 ersed by water in a tall tree is considered, this movement 

 of water through a system of dead cells is a very important 

 fact to explain. 



In most trees, as the mass of wood increases in diameter, 

 the ascending sap abandons the inner (older) wood and moves 

 only through the newer wood. This results in a different 

 appearance of the two regions, the old central wood, aban- 

 doned by the sap, becoming darker and often characteristi- 

 cally colored (heart wood), and the younger outer wood, 

 used by the sap, being lighter colored (sap wood). Trees 

 vary greatly in the relative thickness of the sap wood ; for 

 example, in the beech it is a thick zone, while in the oak it is 

 a narrow one. In successful girdling this must be taken into 

 account, since an incision which would cut off the water 

 supply of an oak sufficiently to kill it would not kill a beech. 



The rate of movement of the ascending sap of course varies 

 with different plants and different conditions. In the pump- 

 kin-vine, in which the movement is very rapid, it has been 



