Coming to the aërial portion, there are all the factors of weather and climate to work singly or in union to affect the above-ground structure of the plant. Temperature varies through wide ranges of heat and cold, scorching and freezing; while humidity or aridity, sunshine or cloudiness, prevailing winds or sudden tornadoes all have an influence in shaping the structure, developing the part, and fashioning the details of form of the aërial portions. Phytoecology deals with all these, and includes the consideration of that struggle for life that plants are constantly waging, for environment determines that the forms best suited to a given set of conditions will survive. This struggle has been going on since the vegetable life of the earth began, and as a result certain prevailing conditions have brought about groups of plants found as a rule only where these conditions prevail. As water is a leading factor in plant growth, a classification is made upon this basis into the plants of the arid regions called xerophytes. The opposite to desert vegetation is that of the fresh ponds and lakes, called hydrophytes. A third group, the halophytes, includes the vegetation of sea or land where there is an excess of various saline substances, the common salt being the leading one. The last group is the mesophytes, which include plants growing in conditions without the extremes accorded to the other three groups. This somewhat general classification of the conditions of the environment lends much of interest to that form of field botany now under consideration. As the grouping is made chiefly upon the aqueous conditions, it is fair to assume that plants are especially modified to accommodate themselves to this compound. Plants, for example, unless they are aquatics, need to use large quantities of water to carry on the vital functions. Thus the salts from the soil need to rise dissolved in the crude sap to the leaves, and in order that a sufficient current be kept up there is transpiration going on from all thin or soft exposed parts. The leaves are the chief organs where aqueous vapor is being given off, sometimes to the extent of tons of water upon an acre of area in a single day. This evaporation being largely surface action, it is possible for the plant to check this by reducing the surface, and the leaf is coiled or folded. Other plants have through the ages become adapted to the destructive actions of drought and a dry, hot atmosphere, and have only needle-shaped leaves or even no true ones at all, as many of the cacti in the desert lands of the Western plains. Again, the surface of the plant may become covered with a felt of fine hairs to prevent rapid evaporation, while other plants with ordinary foliage have the acquired power of moving the leaves so that they will expose their surfaces broadside to the sun, or contrariwise the edges only, as heat and light intensity determine.