Embryology of Macrosolen cochinchinensis

Abstract

1. The flower of Macrosolen is hexamerous. There is an infundibuliform calyculus, a long perianth tube which opens at the tip into six highly reflexed lobes, and six epiphyllous stamens. The ovary is inferior, and from its base there arises a trilobed projection, the mamelon, which reaches up to the junction of the ovary and the style. 2. The vascular supply of the flower is based on a hexamerous plan. The perianth tube receives six vascular bundles, each of which divides to form three to five strands. The stamens receive one vascular bundle each, and three bundles enter the style. 3. The anther wall is formed of the epidermis, an endothecium, one middle layer, and a single-layered tapetum composed of binucleate cells. 4. The microspore mother cells divide simultaneously and form tetrads of the tetrahedral or decussate type. 5. The pollen grains are shed in the two-celled condition. 6. There is a one- to four-celled archesporium which arises in the hypodermal layer of each of the three lobes of the mamelon. 7. Linear tetrads of megaspores are formed. The basal megaspore generally functions. Development proceeds normally up to the four-nucleate stage of the embryo sac. 8. All four nuclei do not divide simultaneously. The two basal nuclei divide first, so that a six-nucleate stage intervenes before formation of the final eight-nucleate stage. The three antipodal cells are organized at the six-nucleate stage. The synergids are ephemeral, but their degenerated remains persist even up to formation of the four-celled proembryo or later. 9. During maturation of the embryo sac the cells at the base of the ovary just below the mamelon develop collenchymatous thickenings. They are laid down in the form of a deep cup or test tube, which is filled with thin-walled cells rich in food material. 10. The upper end of the embryo sac comes to lie in the apex of the mamelon. From the lower end a caecum or lateral haustorium grows into the tube of collenchyma, while the antipodal cells degenerate in situ. 11. Double fertilization occurs normally. Triple fusion takes place just below the egg. 12. The first division of the zygote is longitudinal and is followed by a series of transverse divisions resulting in a biseriate proembryo with an extremely elongated suspensor which causes the embryo to pierce the endosperm and enter the tube of collenchyma. 13. Generally only one of the three embryos formed in an ovary reaches down to the bottom of the tube. This embryo alone reaches maturity, while the others are crushed and absorbed. 14. Soon after touching the bottom of the collenchyma tube the embryo is retracted into the endosperm by the coiling of the suspensor. 15. The mature embryo possesses two free cotyledons, a small conical plumule, and a swollen radicle. 16. The endosperm is cellular, and the endosperms of all the embryo sacs fuse to form a composite mass which grows vigorously and consumes the mamelon. At its upper end it becomes separated into six toothlike structures. 17. The seed is naked, and the endosperm lies directly within the fruit wall, which is formed of four regions of cells. As in other members of the Loranthoideae the viscid layer is very well developed and is responsible for ejection of the seed and its adhesion to the substrate.