DEVELOPMENTAL ANATOMY OF SECRETORY CAVITIES IN THE MICROSPOROPHYLLS OF GINKGO BILOBA L.

Two to five secretory cavities develop in the hump region of the microsporophylls of Ginkgo biloba. A developing cavity is first recognized as a spherical pocket of large, densely cytoplasmic cells (central secretory cells) in the median portion of a microsporophyll primordium. These cells degenerate and a small cavity is formed which is filled with the contents of the degenerating cells. Flattened incurved cells (parietal secretory cells) develop around the disintegrating central secretory cells and slough off into the enlarging cavity. Thus, the cavities develop by lysigeny. A mature cavity is surrounded by senescent parietal secretory cells, scanty parenchyma, and a loosely fitting epidermis. Histochemical tests indicate the presence of lipid and pectic substances in the cavities. Previous reports on the morphological interpretation and possible function role of the cavities are discussed in the light of the present investigation. INTERNAL SECRETORY STRUCTURES are a prominent anatomical feature in Ginkgo biloba. They occur as ducts of various lengths in stems, leaves, and bud scales, or as more or less isodiametric cavities in embryos and strobili. Previous investigations on the development and content of the secretory spaces in Ginkgo have been contradictory. Tunmann (1905) reported a schizo-lysigenous development, while Sprecher (1909) and Starr (1910) suggested lysigeny. Scholz (1932) reported that the ducts (in leaves) contain primarily fats, oils, and resins. However, Starr (1910), Bierhorst (1971), and Foster and Gifford (1974) refer to the secretory cavities (in the microsporophylls) as mucilage cavities. Starr (1910) also suggests that the cavities in the microsporophylls represent abortive sporangia, because of an apparent similarity between early cavity ontogeny and that of normal microsporangia. The present report represents part of an anatomical and ultrastructural investigation of the secretory cavities in the microsporophylls of Ginkgo. It deals with the anatomical development of the cavities and the histochemistry of their contents, and attempts to clarify the contradictory interpretations found in the existing literature. It also serves as a preface to a later report dealing with electron microscope investigations on cytological aspects of the secretory process in Ginkgo. MATERIALS AND METHODS-Male strobili were obtained from large trees at various times throughout the year, beginning in mid-July (the I Received for publication 11 September 1979; revision accepted 4 January 1980. time of strobilus initiation) to the time of strobilus abcission (mid-May of the following year). Whole strobili, parts of strobili, or individual microsporophylls were dissected from buds and fixed for 3-4 hr at room temperature in 4% glutaraldehyde in .15 M cacodylate buffer (pH 7.2), followed by three washes in buffer and post-fixation in 2% osmium tetroxide for 2 hr. Washing, post-fixation and dehydration were carried out at 1-2 C. Tissues were embedded in Spurr's medium, sectioned at 2-3 ,um with a steel knife on a rotary microtome (Ameele, 1976) and stained with the methylene blueazure A-safranin combination of Warmke and Lee (1976) for general observation. Selected histochemical tests were applied either to fresh cavity contents or to FAAor Craf-fixed, paraffin-embedded material (Jensen, 1962). RESULTS-The mature male strobilus of Ginkgo consists of numerous microsporophylls attached to a central axis. Each microsporophyll consists of a microsporangiophore, terminated by a hump of tissue containing the secretory cavities. The two pendant microsporangia are suspended from the hump (Fig. 1, 12). Five to ten strobili are initiated within the buds of the spur shoots in mid-July. A strobilus at first appears as a small mound-shaped structure. Within a few days small protrusions-the microsporophyll primordia-are initiated at the base of the strobilus (Fig. 2). Additional primordia are initiated acropetally, eventually covering the entire surface of the strobilus. Each primordium begins as a rounded protrusion composed of a protoderm layer (as judged by predominately anticlinal divisions) overlying a small mass of densely cytoplasmic cells