THE PATH OF LIQUIDS IN THE PLANT 67 



occupying most of its area. The cell-walls of the xylem 

 are thicker than those of the other cells of the bundle, 

 and have begun to be transformed into wood, hence the 

 name xylem. 2. The phloem, at the opposite, or blunt 

 end of the bundle. The phloem forms part of the bark. 

 3. The cambium, between the xylem and phloem, com- 

 posed of extremely thin-walled cells, and the narrowest 

 of the three regions. The cambium is embryonic tissue, 

 with its cells in a state of active division. The new cells 

 formed next the xylem soon become transformed into 

 xylem-cells; those formed next the phloem, into new 

 phloem. The cambium itself persists throughout the 

 life of the plant. It is perpetual embryonic tissue, never 

 becoming entirely transformed, but giving rise to new 

 cells on either side so long as the plant remains alive. A 

 strand of cambium extending between the bundles {inter- 

 fascicular cambium) gives rise to new bundles, as well as 

 to new fundamental tissue. In time the bundles increase 

 in thickness, and become so numerous and close together 

 that there is an almost continuous cylinder of wood in- 

 side the cambium, and a cylinder of phloem and other 

 tissues outside the cambium. 



65. Passage of Liquids through the Stem.^ — The water 

 and dissolved mineral substances, taken in by the root- 

 hairs, pass up through the xylem to the leaves, while the 

 plant-food, manufactured in the leaves, passes down 

 through the phloem, and is distributed to all living 

 tissues. The liquids passing through the stem are 

 popularly called "sap." 



66. Economic Value of Maple Sap.-^In the case of 

 the sugar-maple, a very sweet sap flows in unusually large 

 quantities during the early spring period of alternate 



