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Journal Article

Contributions to the Life History of a Cycas from Mysore (India)

B. G. L. Swamy
American Journal of Botany
Vol. 35, No. 2 (Feb., 1948), pp. 77-88
Stable URL: http://www.jstor.org/stable/2437890
Page Count: 12
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Contributions to the Life History of a Cycas from Mysore (India)
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Abstract

A species of Cycas, occurring in a natural state in the Hassan District of Mysore State. India, is the basis for the present study. Although differing in some respects from the typical Cycas circinalis L., the plants are tentatively referred to this species. This paper contains a fairly complete description of the so-called "life history" of this cycad. In general, it departs little from the well-known story for other cycad genera. This cycad, in its habitat, shows a well continuing dichotomy of branching of the trunk and a leaf-shedding condition at the time of bearing the cones, both of which happen to be characters not well shown elsewhere in cycads. The microsporangium has a massive stalk and a wall composed of five-six layers of cells. The pollen grains are three-celled at the time of shedding. After pollination, the pollen grain germinates, the tip of which invades the nucellus and becomes haustorial, while the base carrying the prothallial, stalk and body cells becomes pushed down nearer to the archegonial chamber. The body cell divides into two sperm mother cells, within which sperms are organized. The mature sperm is top-shaped and has five-six twists of the ciliated band. The development of the ciliated spiral band from the blepharoplasts has been described in fair detail. The young female gametophyte is free-nuclear; as a result of wall formation in a centripetal manner, it becomes completely cellular. The pollen chamber begins to be differentiated at the free nuclear stage of the gametophyte. The development and structure of the archegonium are typically cycad-like but the number of neck cells may occasionally be four. The ventral canal nucleus is completely disorganized by the time of fertilization. The sperm enters the egg cell between the neck cells and becomes imbedded in the superficial layers of the cytoplasm at the apex. Later, the nucleus of the sperm slips out of the ciliated band and moves downwards to the egg nucleus to fuse with it. Supernumerary sperms were frequently observed in the egg cell and their behaviour is recorded. The early divisions of the zygote are intra-nuclear and show a fibrillar zone around the spindles. After the 64-nucleate stage, the nuclei become distributed throughout the proembryo, but are more crowded towards the base than at the apex. The vacuoles in the central region of the proembryo enlarge and fuse with each other and a large cavity is formed; at the base of this cavity, the nuclei are more numerous and compactly arranged, and cell formation takes place at this end and results in the formation of a suspensor and embryo. Sections of mature ovules show an embryo with two cotyledons and a highly coiled suspensor, which is still attached to the persistent egg membrane.

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