Especially rapid is cell division in growing shoots. The tissues are soft. There is nothing woody nor inert there. The shoot is practi cally pith and cambium. Its cells are rich in leaf green. They need not wait fur the leaves to send them food. They can prepare it unaided. The delicate epidermis is no restraint upon the entrance of carbon dioxid. '('lie cells divide in ally plane. They lengthen as well as thicken. Only at the season's close, when the tissues are ripened to resist winter cold. do they give up once for all the power to grow in length. In all older parts of the tree the bark is rigid and the wood is ri;rid. 1Vhat chance or what use for the cambium to lengthen? I have spoken before in emphatic denial of the common notion that the trunks of trees grow gradually in length as well as in girth. This can be tested by driving nails into the trunk at equal distances, say a foot apart. At the end of two or three years the nails may be swallowed up to the heads by the thickening of the trunk, but they will still be a foot apart. Willow posts grow, hut the wires of the fence are never drawn up and apart. Lower branches die and fall. and their stubs are covered by the hark. They are not lifted up. as the tree grows.
It will be interesting to consider the way the cambium layer forms wood and bark. Constant multiplication of cells takes place. but the thickness of the cambium proper remains constant. It is a single laver of dividing cells which are ever adding to the bark on one side and to the wood on the other. The continual accession of these cells on both sides makes a belt of wood and another of bark. These two belts form a season's growth for the trunk of a tree.
Follow a superannuated cambium cell toward the bark. It is at first a soft cell lying against the inside bark. It has almost enough vitality to divide, hut not quite. Other cells constantly accumulate between it and the cambium. It is forced to give its soft contents to the stream of sap that flows downward through the inner hark. By reason of the growing ring of wood the cambium must grow larger and the bark must stretch. When it can stretch no longer it cracks, and our bark cell feels the outer air. It dries and becomes a
tough bark fibre. The outer layers peel off; the inner ones are pushed out. Filially. after some years. the cell we have been following will reach the surface and be cast off with others of its own age. as a small fibre in a scale of bark.
Let us follow a cell on the other side of the cambium. It lies against the youngest fibres of the sap wood. It loses its cell contents by helping to pass crude sap from the roots to the leaves. New cells form in front of it, separating it ever farther and farther from the life of the tree. It is a fibre now, and when the main sap currents flow up through younger wood this fibre may serve as a storage cell fur starch. its walls may harden, or become dark-colored, by mineral deposits ; they may thicken until little space is left inside. This is when years have elapsed, and several layers of wood lie between the abandoned cell told the cambium. It is a part of the tree's heart wood. But it will never get dry as did the bark fibre; always there is moisture enough to saturate the walls of the wood fibres. Whoevc-nr has burned green wood knows that sap oozes out of heart wood as well as sap wood. though in less (inantity. Ile will remember how dry the bark is, and how the most of the water stews out just under the bark—the place where the cambium is.
Our cell, once a part of the cambium of a sapling, is now buried in the heart of a big tree. But it has never changed its place. It is just as far from the pith as ever. But each annual layer of wood buries it deeper from the outside of the tree.
Tlie growing season for most trees ends before mid-summer. The leaves are battered and eaten by insects. The summer droughts cut off the supply of water. The buds are to be prepared for winter; the shoots must be hardened; the new wood must be ripened. Prepar ation for winter takes the place of further thickening of trunk or lengthening of limb. Twigs and stems and roots are stored with food, the tree tries to take in all the nutritious parts of each leaf before it casts it off. When winter comes it generally finds the tree ready.