"You explained to us, too, sir, that when the tree was young this pith was soft, greenish, pulpy matter, and took up all the center of the stem. It was through this central pith that the dissolved earth-food absorbed by the roots rose upwards to the leaves. It had then only onewoody layer round it. Next year, however, and each year after, a new layer of wood was formed on the outside,and these rings of ne w wood compressed the pith, till at last it became a mere thread. The woody stems of all di-cotyledons grow in this way, by the yearly addition ofa new layer on the outside. They are called ex-ogens. The word exogen means growing outwards.""Very good, my boy," said Mr. Wilson. "Now, Willie shall tell us all he can about the stem of the mono-cotyledon.""The woody stems of these plants are best understood,sir," said Will, "by examining a piece of cane or bamboo.
The wood is always arranged in parallel fibers through the stem from root to top. There is a pith, but it is mixed up with the parallel threads of wood. The parallel bundles of woody threads pass upwards through the pith itself.