EXTERNAL .ANATOMY. The formation of ex ternal organs depends on the unequal growth of the cells composing the body, or of definite groups of them. Thus arise lobes or segments having their own special forms. In the simplest algae and fungi the external segmentation of the body is slight or wanting. The entire body may be represented by a single spherical or oblong cell. Sometimes these cells are branched. In certain desmids the branching is elaborate, and so sym metrical as to make the body an object of great beauty. In all these cases, however, the branch has, in it self, a structure precisely like the main body. In somewhat more complex plants the body consists of a row or filament of cells. Among these plants it is very common to find branches arising which are themselves branched, and repeat in all essential characters the main axis. Other plants have their cells arranged in
the form of a flat plate. This plate may become cause of the unlike conditions under which the two exist. However little or much the body may be lobed, there will he an unequal exposure to light, and the side best illuminated, whether of organ or whole plant, will take on a different structure from the shaded one. Thus the whole body of liverworts and the leaves of seed plants become dorsiventral. Other factors also deter mine the mode of growth; e.g., an erect position and the consequent exposure of the body to the loss of water demands organs for absorption. for conduction, and .for protection against excessive evaporation; again, the cells in the interior, re moved from the air, must be supplied with it by the development of an aerating system.