THE TREE

Trees and Men

Live oak near Hahnville in Louisiana: "I am the gift of God and friend of man."

Pictured above is the famous Logan Elm, in southern Ohio. State-owned, it antedates the Revolution. In 1939, the tree was 70 feet high and had a crown spread of 148 feet.

A TREE IS A LIVING THING

N. T. MIROV.

FROM THE SEED that in the autumn falls to the ground and is covered with leaves and soil, a tree is born. The seed is a thing to marvel at.

Pick up a pine nut; crack it open. The rich kernel, called endosperm, is packed with starch, fat, and proteins. Inside the kernel is cradled the ivory rod that is an embryo pine, a baby tree. On one end of the miniature stern is a tuft of pale leaves; the tapering opposite end of the rod will develop into a root.

Cut open a mellow acorn. In it the baby tree does not rest inside rich, nutritional tissue. The starch and fat and proteins are packed in the two seed leaves of the embryo, which are Plump and round like the two halves of a peanut. The whole acorn inside the shell is an embryo.

In the spring, when the soil gets warm enough and moisture is abundant, deep changes begin to take place in the dormant seed, already conditioned by the low winter temperatures. The embryo tree awakens from its sleep and begins to grow. What causes this awakening of life is not exactly known, and what is known is complicated, indeed. The growth hormone is activated; the enzymes, whose part is to direct and hasten living processes, start their work feverishly. The insoluble stored fats and starch begin to break down to soluble sugars, mainly dextrose. The stored proteins are split by the enzymes into some 20 soluble compounds called amino acids. Both sugars and amino acids are rushed to the growing points, where still different enzymes rearrange them into building material to be used by the germinating embryo. Proteins are formed again from the amino acids, and dextrose is partly used for building the body of the tree and partly burned up to provide necessary energy for the process.

The embryo grows fast. Soon the seed shell becomes too small and splits open. The newly born tree emerges above the ground. Its shoot begins to grow straight up and its roots straight down. The root has important work to do; it provides water for the young seedling. As soon as the little root of a seedling penetrates the ground, the tree is permanently anchored, for better or for worse, to the place where, unless it is transplanted, it has to stay all its life. From now on the tree has to depend on the nutrients available in that particular place and to develop under climatic conditions found there, which cannot be changed. In nature, however, a seedling generally begins its life in a place where its ancestors have been growing for a long time, so the little tree is well adapted to the existing conditions.

As it emerges from the ground, a young tree seedling is as tender as a blade of grass. Its seed leaves may remain in the shell below the ground, as in oak, or they may be carried above the ground, as in maple. In pine, the seed leaves pull themselves out from the endosperm and spread above the seedling like the crown of a miniature palm tree. On the tip of the little stem, tucked between the seed leaves, is the growing point or terminal bud that gives origin to the shoot; its growth continues as long as the tree lives.

Besides the root and stem tips, another important growing region is soon established in the seedling. It is called the cambium layer and is found between the wood and the bark. It makes the tree grow in girth. The cambium consists of a single layer of cells that retain their capacity to divide throughout the life of the tree. This single layer of cells has a peculiar property in that it gives origin both to the wood and to the bark. In the spring, when the cambium layer becomes active, it begins to split off rows of wood cells to the inside and rows of bark cells to the outside. Generally speaking, the bark part of the tree is much thinner than the woody part, or the stem. Bark continuously sloughs off, while the wood accumulates. In the soft inner bark, or bast, are formed sieve tubes, through which manufactured sugar dissolved in water flows from the foliage to storage tissues in stem and root.

The wood formed in the spring consists of light-colored, thin-walled cells; toward the end of the season smaller cells are formed their walls are heavier and darker, and thus summer wood is formed. This alternation of spring wood and summer wood causes the concentric structure of the tree trunk known as annual rings; they are seen clearly on the cross section of a tree. By counting the annual rings of a tree, one can determine fairly closely its age. When growth conditions are favorable and food and water are abundant, the rings are wide. When drought occurs, the growth slows down and the rings are narrow. By reading a cross section of an old tree, one can determine what growth conditions prevailed during any particular year of the past.