=Assimilation.=—_The food from the air and that from the soil unite in the living tissues._ The “sap” that passes upwards from the roots in the growing season is made up largely of the soil water and the salts which have been absorbed in the diluted solutions (p. 67). This upward-moving water is conducted largely through certain tubular canals of the _young wood_. These cells are never continuous tubes from root to leaf; but the water passes readily from one cell or canal to another in its upward course. The upward-moving water gradually passes to the growing parts, and everywhere in the living tissues, it is, of course, in the most intimate contact with the soluble carbohydrates and products of photosynthesis. In the building up or reconstructive and other processes it is therefore available. We may properly conceive of certain of the simpler organic molecules as passing through a series of changes, gradually increasing in complexity. There will be formed substances containing nitrogen in addition to carbon, hydrogen, and oxygen. Others will contain also sulphur and phosphorus, and the various processes may be thought of as culminating in =protoplasm=. _Protoplasm is the living matter in plants._ It is in the cells, and is usually semi-fluid. Starch is not living matter. The complex process of building up the protoplasm is called =assimilation=. =Respiration.=—_Plants need oxygen for respiration, as animals do._ We have seen that plants need the carbon dioxide of the air. To most plants the nitrogen of the air is inert, and serves only to dilute the other elements; but the _oxygen is necessary for all life_. We know that all animals need this oxygen in order to breathe or respire. In fact, they have become accustomed to it in just the proportions found in the air; and this is now best for them. When animals breathe the air once, they make it foul, because they use some of the oxygen and give off carbon dioxide. Likewise, _all living parts of the plant must have a constant supply of oxygen_. Roots also need it, for they respire. Air goes in and out of the soil by diffusion, and as the soil is heated and cooled, causing the air to expand and contract. The oxygen passes into the air-spaces and is absorbed by the moist cell membranes. In the living cells it makes possible the formation of simpler compounds by which energy is released. This energy enables the plant to work and grow, and the final products of this action are _carbon dioxide and water_. As a result of the use of this oxygen by night and by day, plants give off carbon dioxide. _Plants respire; but since they are stationary, and more or less inactive, they do not need so much oxygen as animals do, and they do not give off so much carbon dioxide_. A few plants in a sleeping room need not disturb one more than a family of mice. It should be noted, however, that germinating seeds respire vigorously, hence they consume much oxygen; and opening buds and flowers are likewise active.