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How Trees Breathe

tree, oxygen, carbon, dioxid and cells


When we say- a tree fir/.P thus and so. or it thinks this or that, we are indulging in fancy. lInt when we say a tree bruttlics, we state an accepted scientific truth. There is no make-believe about it. A tree inhales oxygen and exhales carbon dioxid. Even before its life as a tree began, while vet it lay as an embryo in a dry seed, it was 1)reathing. Else the seed could not have germinated. 13reathing is a serious busi ness. It begins with life's beginning, and lasts till its close. Night and day, winter and summer, year after year, the tree breathes. So do all other living things, plant and animal. Respiration is the fundamental operation upon which all other life functions depend.

The life of a plant or an animal resides in its living cells. They feed and grow and divide. The energy for these activities is gener ated by the chemical union of free oxygen with carbon. This union results in a tearing down of molecules of the cell substance. The damage to the cell is repaired by the assimilation of food, and the united carbon and oxygen in the form of carbonic acid gas passes out of the tree as waste.

The breathing of a tree is process by which oxygen is brought into contact with its living cells and the carbon dioxid cast out. It is a simpler matter than the breathing of animals, for most of the living cells of a tree are near the surface, while in animals they are distributed through the body and the oxygen has to be sent to these cells, and the carbon dioxid removed from them, by the blood. The wood of a tree is not alive. Neither is the bark. But between wood and bark, from tip of root to tip of twig is the eanzhiuin, which is the living part of the tree. This living layer is ministered to (as explained in the chapter, I low Trees Feed,'') and it builds new wood and bark. it lengthens the branches and the roots. It adds to the tree's diameter.

The leaves may be regarded as extensions of cambium. They have been called the lungs of the tree. It is tine that oxygen enters the tree chiefly through the little openings, or doorways, called stoma/es, which are located usually on the lower surface of the leaf. On the leaves of apple trees there are estimated to he one hundred thousand of these tiny open ings to the square inch. These doors admit air into the loose tissues; thus the oxygen comes into contact with the livimg cells; and through these opeitin.;.8 the carbon dioxid escapes.

The lungs of animals are not equal to their task unless helped by the skin; and the leaves of plants are not able to du the work of breathing without help. Pale dots and lines inay be plainly seen on the twigs and smooth brunches of most trees. These are called Iwtticiis. They are openings in the young bark. 1;v these pores oxygen gains admission to the cambium. and through them carbon dioxid escapes. The cracking of the bark usually obscures these lenticels un the trunk and older branches. but on certain smooth-barked trees, as birch and cherry, we may still see them even on the trunk. To some extent the cracking and

scaling of the bark as it grows old admits air to the living cells beneath it.

Underground, the roots are active in taking oxygen from the air which is present in the porous soil. This air is not stagnant, but is in constant, if not very rapid, circulation among the particles of soil, to a moderate depth. A tree is often killed by the filling in Of earth above it when the land is being graded to higher level. Also a tree may be killed by a change that keeps its roots water-soaked. Both these changes are injurious because they interfere with the breathing of the roots. The first brings death by smothering; the second. death by drowning. Coal gas leaking from pipes running among the routs often kills a tree. It is a case of choking to death.

As it has no power to move about, a tree does not need to breathe very vigorously. It consumes less oxygen in proportion to its size than nit animal does, and gives out less carbonic acid gas. A young tree breathes more vigorously than an old one. in early summer the grow ing season is at its height; the cells are most active, and the demand for oxygen is greatest. The tree breathes deeply. 1>> winter the activity of the cells is practically suspended, and little oxygen is needed. The tree sleeps. and its breathing is low. The day's work of a tree ends at sun clown. Therefore the demand for oxygen is less by night than by day.

We often hear it said that a plant "breathes in carbon dioxid and -breathes out" oxygen. This is inaccurate. It is true that a plant takes in dioxid, but this process is /ee(litlq. not breathing. As with us, plants feed at intervals; they breathe without ceasing as long as they live. A better statement would lie this: plants breathe oxygen and exhale carbon dioxid just as we do; in the process of feeding, they take in carbon dioxid and give out oxygen.

The perspiration of plants is known as tranyirolion. It is the exhala tion of water, and it takes place mainly through the stomates of the leaves. crude sap is mostly water. and the amount brought to the leaves is in excess of the amount necessary to flow back through the cambium as digested sap, even counting out the water used in the making of starch by the leaves. Night and day the excess of water is being thrown out, though we cannot see it. It passes as an invisible vapor into the atmosphere. Just so does moisture pass out from the lungs and skin of animals contimiall \-. Cover a plant with a bell jar, and we soon see the moisture gradually accumulating on the inside of the jar. The amount of this sweat" depends upon the leaf area of the plant. Botanists estimate that for every ounce of dry matter produced, fifteen to twenty-five pounds of water must pass through the plant. "A large oak tree may transpire one hundred and fifty gallons of water per day dur ing the summer."