Megagametophyte development in Hordeum vulgare. 2. Later stages of wall development and morphological aspects of megagametophyte cell differentiation

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Abstract
The micropylar quartet of nuclei in the barley megagametophyte is first partitioned by a vertical wall between the synergid nuclei and by an initially horizontal wall between the micropylar polar and egg nuclei. The latter wall continues to grow in an expanding horizontal plane forming much of the upper wall of all three egg apparatus cells and eventually fusing with the megagametophyte wall peripherally. A branch of the egg-polar nucleus wall grows in a micropylar direction and becomes attached to the megagametophyte wall. After partitioning, the egg apparatus is composed of three flat cells having a ceiling wall and two upright supporting walls, which are fused centrally. The micropylar polar nucleus lies just chalazal to the ceiling wall. Expansion of the egg apparatus results in rounding of all three cells followed by lengthening and thinning of their walls in contact with the central cell. Probable membrane contacts may facilitate sperm transmission after pollination. Partitioning of the chalazal quartet of nuclei exhibits many similarities to that of the egg apparatus but with a different cellular arrangement. Transfer cell wall ingrowths appear in cells at both poles of the megagametophyte. Such ingrowths appear in the two synergid cells, representing the filiform apparatus. They also develop in two of the original three antipodal cells where these cells are in contact with the megagametophyte wall. Either the micropylar or chalazal polar nucleus migrates to a position close to the other polar nucleus. Partial fusion of polar nuclei occurs later.