—Plants, like animals, breathe; like animals they breathe in oxygen and breathe out carbonic acid gas. Respiration which is but another name for breathing, goes on day and night, but is far less active than assimilation, which takes place only during the day. Consequently more carbonic acid gas is taken in than is given out except at night when, to a slight extent, the reverse takes place, small quantities of carbonic acid gas being given off and oxygen taken in.
Very small openings in the bark called lenticles, furnish breathing places. Oxygen is also taken in thru the leaves.
Transpiration is the evaporation of water from all parts of the tree above ground, principally from the leaves.
The amount of water absorbed by the roots is greatly in excess of what is needed. That fresh supplies of earthy matter may reach the leaves, the excess of water must be got rid of. In trees with very thick bark, transpiration takes place thru the lenticles in the bottom of the deep cracks.
116. Moisture.
—Water is present in all wood. It may be found (1) in the cavities of the lifeless cells, fibers and vessels; (2) in the cell walls; and (3) in the living cells of which it forms over ninety per cent. Sapwood contains more water than heartwood.
Water-filled wood lacks the strength of wood from which the greater part of the moisture has been expelled by evaporation.
117. Shrinkage.
—Water in the cell walls—it makes no difference whether the cells are filled or empty—causes their enlargement and consequently an increase in the volume of the block or plank. The removal of this water by evaporation causes the walls to shrink; the plank becomes smaller and lighter. Thick walled cells shrink more than thin ones and summer wood more than spring wood. Cell walls do not shrink lengthwise and since the length of a cell is often a hundred or more times as great as its diameter the small shrinkage in the thickness of the cell walls at A and B, in [Fig. 199], is not sufficient to make any noticeable change in the length of the timber.