Histological Study of the Developing Fruit of the Sour Cherry

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Abstract
This paper pictures and discusses the gross development of the fruit of the sour cherry {Primus cera.sus var. Montmorency) from 18 days before full bloom to fruit ripening, and the histological changes-during the pre-bloom stage, stage I (rapid development for 20 days following full bloom), stage II (retarded development for 16 days), and stage III (rapid development for 21 days to fruit ripening). Three principal tissues compose the ovary wall; inner and outer epidermis, stony pericarp, and fleshy pericarp. The stony pericarp may be divided into an inner and an outer layer, and the fleshy pericarp into an innermost layer of small thin-walled parenchyma, a middle region of large thin-walled parenchyma, and an outer or hypodermal layer of collenchyma. The fleshy and the stony pericarp are derived each from distinct groups of cells which are early separated from one another by characteristic size, shape, and frequency and periodicity of cell division. The inner layer of the stony pericarp is derived from the inner epidermis, together with a few adjacent cells of the pericarp, and forms a band or "hoop" of transversely elongated cells bounding the inner ovary wall. The outer layer is derived from the pericarp, and the cells of which it is composed are elongated at right angles to those of the inner layer. The cells of the stony pericarp increase in number during the pre-bloom stage and the first few days of stage I, and enlarge during the latter part of stage I. Cell walls become progressively thicker, and by the end of the period the maximum number and size of cells is attained. During stage II the walls thicken and harden greatly. During stage III there is slight increase in hardness and brittle-ness. The cells of the fleshy pericarp increase in number during the pre-bloom stage and the first half of stage II. In the last half of stage II they double in diameter. During stage II there is slight enlargement. During stage III the size of individual cells increases remarkably. Those in the outer portion of the fleshy pericarp become roundish-oval, with the greatest diam. parallel to the periphery of the fruit; those next inward become roundish; those next, obovate in a radial direction; next, radially elongate; and innermost, decidedly radially elongate. At maturity the largest cells indicate an increase of 25 times in diam. from the size at full bloom. The epidermal cells are elongated radially 18 days before full bloom. They increase rapidly in number during the pre-bloom stage and during the first half of stage I, increase in size and wall thickness during the later half of stage I, change but little during stage II, and greatly enlarge tangentially in stage III. The stomata are fully differentated 18 days before full bloom. The guard cells increase in size as the fruit develops, but the increase-is less than that of typical epidermal cells. The similarity of other fruits and the mechanism of enlargement in stage III are discussed.

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