"RESPIRATION is essentially the intake of oxygen and the output of carbon-dioxide by living cells. In the higher animals two phases of respiration are distinguished the exter-nal, the exchange of gases between the air or water and the blood; and the internal, the exchange between the blood, lymph, and the tissues." I In plants there is, for the most part at least, only the one phase, the exchange of gases between the air or water and the cells composing the tissues, an exchange which is direct and " external," since it takes place in most cases between the air, whether in the intercellular spaces within the plant, or unconfined and outside the plant body, and the individual cells. Even in the densest tissues, within which the intercellular spaces are small, it is likely that the cells take in free oxygen and give out carbon-dioxide, if not directly from intercellular spaces, then from their neighbors bordering on intercellular spaces. In any case, and in every stage of the process of respiration except the purely mechanical ones, of which only the higher animals are capable, the exchange of gases between the cells and the air takes place in solutions, the oxygen entering and diffusing through, the carbon-clioxide passing out from, the cells only when these gases are dissolved in water. The object of respiration in plants is not the maintenance of a certain body temperature, together with the production of energy needed for doing work, as in warm-blooded animals. It is merely the production of energy for doing work, as in cold-blooded animals. The average body temperature of plants is, in general, nearly the mean daily temperature of their environment. It will vary within certain limits, the variation being