WHAT PLANTS ARE) MADE) OF 
49 
mass. At first, all the members of this cell-mass are 
alike, and all do the same work, but as the tree grows 
larger there is more work to be done, and so the dif¬ 
ferent cell-masses prepare themselves to perform their 
several duties. Some are set apart for the drinking in of 
water from the soil; others act as a line of messengers 
to pass the water up the tree for the nourishment of cell- 
masses which lie too far from the soil to gather their own 
water. These messenger cells lie all along up the tree 
trunk, and they are connected one with another by means 
of fine threads of protoplasm. Through these threads 
the sap runs from cell to cell. Still other cell-masses, 
like those which make up the heartwood and bark, exist 
only to give bulk and protection to the tree so that it 
may withstand wind and weather. In the leaf, as you 
know, are the leaf-cells which contain that magic green 
fluid, chlorophyll, which uses sun-light to turn air and 
water into plant food. 
And always these cells are building up and breaking 
down, wearing out and renewing themselves—what did 
we call this process? A big word, but a short while ago 
you said it over five times: metabolism. This continues 
as long as there is life and growth in the tree, which is a 
long time, because even the oldest tree puts forth new 
buds and shoots. 
