Gross Morphology and Histology of Developing Fruit of the Apple

1. Gross development and principal tissue changes in the fruit of the apple from 1 month before full bloom throughout the growing season to fruit ripening are figured and discussed, and suggestions are made regarding the methods and the values of measurements of various parts. 2. Interpretation of the apple as five drupelike carpels contained within the fleshy torus or receptacle has seemed to describe the structures observed. 3. The curve of gross development of the entire fruit is nearly a straight line for the early summer variety (Early Harvest), but for each successively later-ripening variety (McIntosh and Rome) the curve flattens as the season progresses. Although perhaps associated with environment, the rate of growth is shown to be a varietal characteristic, in which each successively later-ripening variety has a slower rate. 4. The cartilaginous portion of the carpels develops rapidly for 2-4 weeks after full bloom, reaching maximum size in transverse diameter the earliest of any of the tissues making up the bulk of the fruit. The fleshy portion of the carpels continues growth approximately 2 weeks longer and is the next tissue to reach maximum size. The pith and cortical regions continue growth up to fruit ripening and constitute the bulk of the apple at that time. 5. The edges of the carpels are in approximate contact along the ventral suture up to about 4 weeks after full bloom. In such varieties as the Twenty Ounce, the edges then begin to spread apart along the ventral suture and curve away from the central axis of the fruit, resulting in an "open core" condition. 6. Development of the carpels resembles somewhat development of a drupe fruit. Each carpel is composed of inner epidermis, cartilaginous pericarp, fleshy pericarp, and outer epidermis. 7. The cells of the inner epidermis are radially elongate 4 weeks prior to full bloom. Following full bloom the cells of the inner epidermis elongate rapidly obliquely toward the apex of the fruit and toward the dorsal carpellary bundles, except for some longitudinal strips in the region of the dorsal and ventral bundles. During the second and third months after full bloom the cell walls become greatly thickened, until at fruit ripening the lumen is scarcely more than a line. 8. The cartilaginous pericarp increases by cell division prior to full bloom and by cell elongation and some cell division after full bloom. During the second month after full bloom the cell walls begin rapid thickening, which progresses until at fruit ripening the lumen is about one-fifth the diameter of the cells. There is an absence of sclerenchymatous cells in the region of the carpel bundles, so that the hardened portions of the pericarp form two disconnected sheets of tissue, one on either side of the locule. As the fruit nears ripening the cartilaginous pericarp may become fissured or split, resulting in a tufted condition when parenchyma tissue grows between the breaks. The gross size of the cartilaginous carpel may increase by this means, even after hardening is complete. 9. The fleshy pericarp may be distinguished from the cartilaginous pericarp at full bloom. Cell division is most abundant prior to and at the time of full bloom. Cell enlargement follows, many of the cells becoming radially elongate and reaching 300 μ at fruit ripening, as compared with 10 μ at full bloom. 10. The pith can be recognized as early as 1 month before full bloom. There is much cell division for 11 days just at and immediately after full bloom. Cell division seems to have ceased by 3 weeks after full bloom. Increase in size of pith is due thereafter to increase in size of cells and of intercellular spaces, some cells reaching 150 x 300 μ. 11. The cortical region prior to full bloom increases rapidly in number of cells, by cell division. During the next 2 weeks there is rapid increase in both number and size. Cell division appears complete 3 weeks after full bloom, so that subsequent increase in size is by enlargement of cells and of intercellular spaces, some cells attaining a size of 197 x 340 μ. 12. Cells of the hypodermal layer can be distinguished by full bloom by slight thickening of the walls. Both tangential elongation and thickening of the walls continue until the fruit has attained full size. 13. The cells of the epidermis are palisade-like and readily distinguished at least 1 month before full bloom. The number of cells is more than doubled between the twenty-fifth and the third day before full bloom. From the third day before full bloom until the twenty-fourth day after it increases more than six times and appears to continue to some degree until 85 days after full bloom. As the period of fruit ripening approaches there is much tangential stretching of the cells to accommodate the increasing surface area. 14. Development of pericarp, nucellus and integuments, and embryo is similar in some details to the stages of development of similar parts in drupe fruits; but the bulk of the apple fruit, being accessory tissue, shows no such stages.