=30. The influence of factors.= While the above classification is both obvious and convenient, a more logical and intimate grouping may be made upon the influence which the factor exerts. On this basis, factors are divided into (1) direct, (2) indirect, and (3) remote. Direct factors are those which act directly upon an important function of the plant and produce a formative effect: for example, an increase in humidity produces an immediate decrease in transpiration. They are water-content, humidity, and light. Other factors have a direct action: thus temperature has an immediate influence upon respiration and probably assimilation also, but it is not structurally formative. Wind has a direct mechanical effect upon woody plants, but it does not fall within our definition. Indirect factors are those that affect a formative function of the plant through another factor; thus a change in temperature causes a change in humidity and this in turn calls forth a change in transpiration; or, a change in soil texture increases the water-content, and this affects the imbibition of the root-hairs. Indirect factors, then, are temperature, wind, pressure, precipitation, and soil composition. Remote factors are, for the most part, physiographic and biotic: they require at least two other factors to act as middlemen. Altitude affects plants through pressure, which modifies humidity, and hence transpiration. Slope determines in large degree the run-off during a rainstorm, thus affecting water-content and the amount of water absorbed. Earthworms and plant parts change the texture of the soil, and thereby the water-content. Indirect factors often exert a remote influence also, as may be seen in the effect which temperature and wind have in increasing evaporation from the soil, and thus reducing the water-content. This distinction between factors may seem insufficiently grounded. In this event, it should be noted that it centers the effects of all factors upon the three direct ones, water-content, humidity, and light. If it further be recalled that these are the only factors which produce qualitative structural changes, and that the classification of ecads and formations is based upon them, the validity of the distinction is clear. THE DETERMINATION OF FACTORS