Comparative Histology of Naturally Parthenocarpic, Hormone-Induced Parthenocarpic, and Caprified Fig Syconia

Abstract

1. Histological comparisons have been made of the following kinds of fig syconia sampled three times during the growth period: (a) naturally parthenocarpic Kadota, (b) caprified Kadota, (c) parthenocarpic Calimyrna induced by parachlorophenoxyacetic acid (PCPA), (d) parthenocarpic Calimyrna induced by gamma-(indole-3)-n-butyric acid (IBA), and (e) caprified Calimyrna. 2. Epidermal cells of Calimyrna induced parthenocarpic syconia at maturity were rectangular in radial section, with their long axes parallel to the surface of the syconium. Epidermal cells of Calimyrna caprified syconia, on the other hand, were rectangular in section, with their long axes perpendicular to the surface of the syconium. Epidermal cells in Kadota naturally parthenocarpic and caprified syconia were observed not to be different from each other but were like epidermal cells of Calimyrna induced parthenocarpic syconia. 3. The cuticle on the inner and outer epidermis of the receptacle, pedicel and perianth of the flowers, outer ovary wall, and integuments of both Calimyrna and Kadota caprified syconia was thicker than in Calimyrna and Kadota parthenocarpic syconia. 4. At each sampling, the thickness of the cortical parenchyma tissue and the number of cell layers contained in it were greater in most cases in both induced parthenocarpic Calimyrna and naturally parthenocarpic Kadota syconia than in syconia that had been caprified. 5. Six to seven layers of cells in the peripheral region of the cortex in both parthenocarpic and caprified Kadota syconia were identified as collenchyma. This characteristic is thought to be responsible for the superior canning qualities of the Kadota variety. 6. Although Kadota syconia contained a few more laticiferous ducts than Calimyrna syconia, no difference in size or development of these tissues existed between parthenocarpic and caprified figs. 7. Neither size nor number of the cells in the vascular bundles was markedly different in the various kinds of syconia examined. 8. The length and width of exocarp cells in the achenes of parthenocarpic syconia were considerably smaller than corresponding cells in caprified syconia of both varieties. 9. The length and width of cells composing the outer mesocarp, as well as the thickness of this tissue, were smaller in induced parthenocarpic Calimyrna syconia than in caprified syconia, whereas the converse was true in the Kadota variety. 10. The development of the inner mesocarp, as measured by thickness, was considerably less in the parthenocarpic than in the caprified syconia of the respective varieties. Inner mesocarp cells of parthenocarpic and caprified Kadota and caprified Calimyrna syconia sclerified upon maturity, but no sclerification whatsoever took place in these cells in parthenocarpic Calimyrna syconia induced by IBA or PCPA. The content of calcium oxalate crystals was altered in the inner mesocarp cells in syconia induced with these growth substances. 11. The thickness and number of cell layers in the endocarp were somewhat greater in caprified than in parthenocarpic Kadota syconia. Although the endocarp was present in parthenocarpic Calimyrna syconia induced by PCPA, it remained parenchymatous in character and did not sclerify upon maturity of the syconium. At maturity, endocarp tissue was totally lacking in IBA-induced parthenocarpic Calimyrna syconia. 12. The integuments and nucellar tissue collapsed comparatively early, and there was no embryo or endosperm development in the achenes in parthenocarpic syconia. 13. The relationship between the splitting of maturing syconia and the development of the achenes is discussed. It is concluded that splitting is the result of growth of the fruitlets within the syconium, the degree of splitting being closely associated with certain environmental conditions. 14. The higher percentage of sugar in parthenocarpic than in caprified syconia is probably the result of the larger cortex and lack of seed development in the former. 15. A short discussion points out the specificity of the synthetic growth substances and the diverse reactions they induce. Although it is believed that the hormone which controls the growth of the syconium, in part, may be produced in the ovarian tissue, it is now known that endocarp tissue plays no part in this in IBA-induced parthenocarpic Calimyrna syconia, in which it is lacking.