different structure is apparent. Any lumberman can point out at once “heartwood” and “sapwood” (Figure 78), and his distinctions are just as good as those of the scientist, for he says in these two words as plainly as can be said that heartwood is the oldest and sapwood the youngest. The sapwood is nearer the bark and is honeycombed with passages which serve to carry the sap from the roots to the tree top, while just under the bark is the bright, green, living layer, known as _cambium_, which is renewed each year. The _phloem_ is the carrier for the food made in the leaves to the roots. It is the successive layers of _cambium_, year after year, that gives to tree trunks their annual rings. The age of almost all trees can be reckoned exactly by counting these, one representing a year’s growth, and the tree’s rate of growth estimated from the closeness of the rings. Fires or droughts, perhaps long forgotten, here find a lasting record in rings so close together as to be all but invisible. The part nearest the center of the trunk is the heartwood, usually quite lifeless, yet in its maturity furnishing us with lumber. It may be and often is completely decayed, without injuring the flow of sap or the life of the tree for many years. [Illustration: FIG. 9.--BULB OF THE ONION Showing root and leaf growth.] These two streams of sap, one going up and the other returning to the roots, each in its proper channel, are interspersed with air chambers that extend from the center of the tree out toward the bark, where they end in inconspicuous dots called _lenticels_. It is as though nature had provided an air-cooling device for the constant activity of these diverse currents. These lenticels are prominent on the bark of cherry, but whether obvious or not they are found in nearly all woody stems and insure a constant supply of fresh air to the busy interior. In palms, sugar cane, corn, bamboo, and many other plants there is not any distinction between heartwood and sapwood (Figure 82), and in place of bark there is nothing but an outer rind, harder than the interior tissue. Such stems do not usually rot first at the center, have no cambium, and have no annual rings. This method of growth and structure is associated nearly always with definite leaf and flower forms peculiar to it and differing from most other plants. So fundamental are these characteristics, so uniform their occurrence and so clear are the distinctions between them and other plants that botanists have divided all flowering plants into those belonging to this group or to some others. More will be said of this in the chapter on the Families of Plants and Their Relationship.