The body of nearly all flowering plants can be divided into two systems, the root system which grows below ground and the shoot system which grows above it. The typical root system anchors that plant firmly in the soil, and absorbs water and various dissolved raw materials from it. The typical shoot system consists of a main stem upon which are borne leaves, branch shoots, and sometimes flowers. It should be emphasized that the words stem and shoot are not synonymous, the word “shoot” being a collective term for both stem and leaves. The leaves are the chief food producing organs of the plant, making complex foods in the presence of light by a process called photosynthesis. The stem supports the leaves, displaying them in the light needed for photosynthesis, and also acts as the main channel of communication between the various organs of the plant. Water and dissolved minerals absorbed by the roots are carried in the stem to the leaves and flowers, and foods synthesized in the leaves are conducted away through the stem to other regions of the plant, both above and below ground, where they are either used for growth or else stored for future use. Flowers are organs that are concerned with the reproduction of the species by seeds. With what topic is the passage primarily concerned?