Polar nuclei behaviour in the mature megagametophyte of the natural tetraploid Trifolium pratense (Fabaceae)

摘要

The mature female gametophyte in Trifolium pratense L., a natural tetraploid plant, consists of 7 cells: the egg cell, 2 synergids, the central cell, and 3 antipodal cells. This type of mature female gametophyte is known as polygonum. The central cell occupies the largest portion of the mature female gametophyte and the polar nuclei are situated close to the egg apparatus. The cytoplasm of the central cell is rich in organelles. A large number of rough and smooth endoplasmic reticulum, mitochondria, plastids, dictyosomes, ribosomes, starch, and lipid bodies were observed. Differences were identified in this plant between the sizes of polar nuclei and secondary nuclei generated by the fusion of the former. It was observed that the polar nuclei had fused long before the pollen tube entered the female gametophyte, and that the secondary nuclei in some female gametophytes were degenerated before fertilisation.