栽培甜菜大孢子发生的超微结构

栽培甜菜（Beta vulgaris）的大孢子发生为蓼型。减数分裂时，大孢子母细胞核中出现核液泡，形成联会复合体，细胞壁上有胼胝质加厚，并存在细胞质改组现象。大孢子母细胞减数第1次分裂形成二分体，2个细胞均被较厚的胼胝质壁包裹。合点端的二分体细胞中细胞器丰富，线粒体和质体的形态正常，表明完成了再分化。在大多数情况下，珠孔端的二分体细胞在减数第2次分裂前（或分裂的过程中）退化，合点端的细胞分裂产生大小不等的2个细胞，形成三分体。三分体合点端的大孢子体积较大，发育成单倍体的功能大孢子。

Ultrastructure of Megasporogenesis in Beta vulgaris

Megasporogenesis of Beta vulgaris （2n=18） is of the Polygonum type. Megasporocytes contain nuclear vacuoles before meiosis and synaptonemal complexes at the early stage of meiosis. Redifferentiation of cytoplasmic components occurs, and callose is deposited in the wall during meiosis. The dyad formed in Meiosis I was wrapped by thick callose. The chalazal cell in the dyad contains many organelles, and mitochondria and plastids showing regular shapes, considered a sign of the end of redifferentiation. The micropylar cell in a dyad always degenerates quickly, and only chalazal one finished meiosis II to form two unequal-sized megaspores. The two unequal-sized megaspores plus a micropylar cell in the dyad formed a triad, and the callose disappeared in the chalazal end of the megaspore cell that developed into a functional megaspore.