The rate of penetration of water through heartwood, pressure and temperature remaining constant, shows an initial fairly rapid decrease followed by a gradual decrease to a constant rate. With unseasoned sapwood, the rate of penetration increases to a maximum, after which it slowly decreases. With seasoned sapwood the rate decreases with elapsed time. Pre-soaking for periods up to seven days does not affect the equilibrium rate of penetration. The initial rate is lower with pre-soaked than with air-dry wood. Pre-soaking does not hasten the attainment of an equilibrium rate. Rate of penetration is fairly constant for heartwood specimens of the same kind taken from the same transverse section of a tree. Unseasoned sapwood is more than 200 times as permeable as heartwood from the same tree. White spruce, black spruce and red pine heartwoods show about the same penetrability at low pressures. At higher pressures red pine becomes much more permeable than white spruce at the same pressure. The same is true of cedar, tamarack and balsam. Unseasoned sapwoods show increasing penetrability in the order hemlock, balsam, red pine, white spruce. Observed apparent penetration radially and tangentially through heartwood is less than 9% of that in the longitudinal direction. (Water actually penetrating was in no case greater than 1% of that penetrating longitudinally in the same time through a specimen of the same thickness.) In sap-wood the rate of radial and tangential penetration is less than 2% of that in the longitudinal direction. Tangential penetration of white spruce sapwood is probably greater than radial penetration. Rate of longitudinal penetration increases with decrease in thickness of specimen. A very great increase in rate is noted after the thickness becomes less than one fibre length. This is suggested as a method of obtaining an approximation to the average fibre length. Rate is not inversely proportional to thickness, but decreases more rapidly with increasing thickness owing to loss of pressure head in passing pit membranes. Rate of penetration increases with increase in pressure differential. The rate is proportional to pressure (or slightly greater) in white spruce. The rate increases much more rapidly than proportionately with balsam, tamarack, cedar and red pine. This is due to bulging of thin pit membranes and consequent enlargement of perforations. Magnitudes of pressure and back pressure have no effect on rate of penetration, or time to reach an equilibrium rate, if pressure differential remains constant. Penetrability is not altered permanently by the application of pressure. Rate of penetration increases with temperature. Temperatures above 70 °C. have a permanent effect on the penetrability. Intensive drying of wood increases its penetrability.Sucrose solutions cause a decrease in rate of penetration greater than that expected from viscosity considerations. Molar sodium hydroxide increases the permeability of heartwood specimens of greater than one fibre length, and decreases that of sapwood and very thin sections of heartwood. Molar hydrochloric acid decreases the rate of penetration through heartwood, but has little effect on that through sapwood. Gases penetrate seasoned heartwood and sapwood easily. Pre-soaked heartwood strongly resists penetration by gases. Short lengths of unseasoned or pre-soaked sapwood are fairly easily penetrated by gases. All the evidence points to the absence of any valve action on the part of pit membrane tori.A new theory has been advanced to account for the phenomena ordinarily ascribed to valve action, and also to explain those observations which could not be explained satisfactorily as due to torus valves.
Electromyography of the Respiratory Muscles and Gill Water Flow in the Dragonet
