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 
