AGRICULTURAL CHEMISTRY.
a bug of any of several families of Hemiptera, which obtain their food by sucking the juices of plants through beak like mouths. The largest group is the family consisting of at least 1,500 species, some of which are dreaded pests of cultivated crops and trees. Two well-known species are the box-elder plant-bug (Leptocoris trivittatus), which is especially troublesome in the western States, where the box-elder is largely planted as a windbreak and for timber: and the squash bug (Anasa tristis), which is an incorrigible enemy of squashes, pumpkins, cucumbers, etc. (See SQUASH). 'the include fully 1,300 species, nearly 200 of which are known in America, and of which one of the most widely destructive is the little chinch-bug, a pest upon certain cereal crops. The Pyrrhocoridce is a small family, but contains several serious pests, of which the cotton stainer or red-bug (Dys thereat suturellus) is a pest in the southeastern United States. It feeds upon orange fruits and cotton. Consult Comstock, 'Manual for the Study of Insects' (1895) ' • Smith, 'Manual of Economic (1896) ; Howard, The Insect (new ed., 1914).
PLANT The relation of the plant covering to the surface of the earth is the basis of that division of botany which is called plant geography. This does not confine itself to the geographical dis tribution of plants, as was formerly the case, but comprises all the out-of-door relations of plants to each other, and to their environment. The subject falls somewhat naturally into several divisions with respect to the point of view. Floristic botany deals with the geographi cal distribution of species, and with the charac ter of the plant population of different regions and countries. Ecology concerns itself espe dally with the relation existing between the plant and its environment, and with the group ing of species in particular areas called forma tions, such as prairie, forest, etc. Experimental ecology is merely a phase of the latter, in which changes in the form and behavior of plants are brought about by changing the physical conditions of the environment. It is of great importance because of its bearing upon the origin of species.
Environment and essential points of inquiry in plant geography are the environment, or home of the plant, usually called habitat by botanists, and the plant, either as an individual, or as a member of the vegetation. The habitat is to be regarded as
the cause, the plant as the effect. This is true of the present relation between any habitat and the plants which grow in it only to a certain degree, for probably no plant of the present day owes its entire structure to one habitat. Each habitat, however, does have a modifying in fluence upon the plants in it. This influence will be great where the physical conditions are ex treme, and it will be slight where they are more moderate. In either case this modification will leave its distinctive stamp upon the plant in such a way that one may readily tell whether it grew in pond, meadow, forest or desert. Of the many factors which make up a habitat not all are of the same importance. In some situations the water of the soil will be most important, in others light will be the controlling factor, while in still other places wind will have the most striking effect. Habitats show great differences also in the amount of water, light, wind and other factors. If one will know the causes of plant structure and distribution, it is necessary to look into the habitat with great care, and to determine the relative importance and the amount of each factor.
The factors of a habitat which are most in timately connected with the form of the plant are those that have to do with the water-supply, and with the food-making activities of the leaf. These are water and light. They have a direct influence upon the plant form, while all others affect the structure indirectly, as a result of their action upon the water of the soil, or the air, or upon light. Every habitat comprises the following factors: water-content, humidity, light, temperature, soil, wind, precipitation, physiography, dead vegetation and animals. Of these, water-content, soil, soil temperature, and physiography belong to the soil, and are in con sequence called edaphic. Humidity (air moist ure), light, air temperature, wind, and precipita tion pertain to the air, and are termed atmos pheric or climatic. Dead vegetation and ani mals are biological or biotic factors. Living vegetation has a marked effect upon its habitat, but this is to be regarded more as a reaction than as a cause.