Fertilization in Ginkgo biloba

Abstract

1. At the time of shedding, the pollen grain of Ginkgo usually contains four cells: the first prothallial, second prothallial, generative, and tube cells. After germination of the pollen grain the first prothallial cell aborts, while the generative cell divides, forming the body and stalk cells. During a late stage of development of the body cell, two blepharoplasts are formed, and the nucleus changes from a spherical to a lens-shaped structure. While the nucleus is changing shape, two vacuole-like structures appear close beneath the blepharoplasts, and a conspicuous granule becomes visible beside the nucleus. 2. Chromatin in the nucleus of the body cell is diffuse but condenses into chromosomes during division. Each spermatozoid cell includes one of the blepharoplasts, which changes from a rounded to a crescent-shaped structure and then forms the cilial band. The vacuole-like structure separates from the blepharoplast and moves to the side opposite the cilial tail. Later it becomes impregnated with chromatin and apparently forms the head of the spermatozoid. One half of the granule near the nucleus of the body cell is included in each spermatozoid. The newly formed spermatozoid is flexible and flows out from the pollen tube. 3. Free-nuclear divisions occur simultaneously in early stages, and more than a thousand nuclei are formed before the initiation of wall formation in the megagametophyte. Wall formation proceeds centripetally. In the free-nuclear stage the cells of the spongy tissue of the nucellus surrounding the megagametophyte may be multinucleate, with extremely large polyploid nuclei. This tissue is absorbed by the developing megagametophyte. 4. Archegonial initials, usually two in number, appear superficially at the micropylar end of the megagametophyte, each containing a large central nucleus. Each initial divides periclinally to form a cover cell and an inner central cell. The cover cell divides anticlinally to form two neck cells. The central cell gradually enlarges, while adjacent cells form a well-organized jacket layer. Just before division of the central cell the two neck cells divide, forming four neck cells, which swell to form a hemispherical protuberance. The central cell divides unequally into the small ventral-canal cell and large egg. 5. When the spermatozoid is released from the pollen tube, the egg cell forms a small projection which extends between the neck cells. As soon as the spermatozoid becomes attached to the egg projection, the latter retreats, leaving a path through the neck. Only the head of the spermatozoid, which includes the chromatin, penetrates the egg cytoplasm, where it forms a globular mass. Simultaneously, the chromatin in the egg nucleus forms another group of granules. The union of the two sets of granules is considered to be the actual fertilization. 6. The first division of the zygote is perpendicular to the axis of the ovule. In telophase an obscure cell plate is visible but becomes less evident later. Free-nuclear division continues until 256 nuclei are formed. Wall formation in the proembryo is simultaneous. As the embryo continues to develop, cells differentiate into the cotyledons and the apical meristems of root and stem.