There is great variation in the structure of fruits, such a variation, in fact, that no one fruit has a structure typical of all the other fruits. Each fruit, however, has a pericarp and one or more seeds. The amount and structure of the cells forming the pericarp and the seeds of fruits differ in different fruits, but for each fruit there is a normal amount of, and a characteristic, cellular structure. Nearly all the important medicinal fruits are cremocarps or umbelliferous fruits. The plan of structure of cremocarps is similar, but they all have a different cellular structure. The epidermis may be simple or modified as papillæ or hairs. The secretion cavities may be absent (conium), or, when present, variable in number--cultivated celery seed has six, wild celery seed up to twelve, and anise up to twenty. The vascular bundles may be large or small. The endocarp cells may be two or more layers in thickness. The spermoderm may be thin or thick. The endosperm cells may vary in size and the cell contents may vary. CELERY FRUIT The fruit of celery (Plate 121), like other umbelliferous fruits, is composed of the pericarp and the seed. The pericarp is composed of epicarp cells, mesocarp cells, endocarp cells, and in each rib a vascular bundle. The seed is composed of the spermoderm, endosperm, and embryo. Each of these parts has a characteristic structure. =Epicarp=. The cells of the epicarp (Fig. 1) are papillæ and the outer wall is striated. The papillæ do not show, however, unless the cell is cut across the centre, which is the point at which the papillæ are located. [Illustration: PLATE 121 CROSS-SECTION OF CELERY FRUIT (_Apium graveolens_, L.) 1. Epicarp. 2. Mesocarp. 3. Vascular bundle. 4. Endocarp. 5. Spermoderm. 6. Endosperm. 7. Secretion cavity.] [Illustration: PLATE 122 DIAGRAMMATIC DRAWING OF THE 1. Cross-section of wild celery seed (_Apium graveolens_, L.). 2. Cross-section of cultivated celery seed (_Apium graveolens_, L.).] =Mesocarp=. In the rib part of the mesocarp (Fig. 2) is a vascular bundle, and between the ribs one or more secretion cavities. The vascular bundles are small and are surrounded by irregular-shaped mesocarp cells. The =secretion cavities= (Fig. 7) are oval in form and the tissue bordering the cavity is reddish brown in color. The mesocarp cells around the secretion cavities are more elongated than the other mesocarp cells. =Endocarp=. The endocarp cells are three layers in thickness. These cells are elongated transversely (Fig. 4). =Spermoderm=. The cells of the spermoderm are indistinct, compressed, and dark brown in color (Fig. 5). =Endosperm=. The endosperm cells (Fig. 6) make up the greater part of the fruit. The walls which are common to two cells are thick, non-beaded, and non-pitted, and the cavities of the cells are filled with aleurone grains.