玉米（ＺｅａｍａｙｓＬ．）叶片细脉原生韧皮部筛分子开始分化时，首先出现长的粗面内质网潴泡和增厚的细胞壁，随后在质体中出现其特征性的拟晶体内含物。随着分化的进行，长的粗面内质网潴泡转化为较短的形态，最后聚集成一些小的堆叠并失去其核糖体。细胞核发生退化，但常保持到成熟期的后期，此时的细胞核由双层核膜或某些部位仅由内核膜包被，内含电子致密的不定形染色质团块。随后双层核膜破裂而变得不连续。在细胞核开始退化时，核周腔局部膨大。有的膨大核周腔的外核膜破裂，并伴随邻近的部分细胞质解体。在核退化过程中，除内质网外，质体和线粒体的结构也发生变化，而核糖体、细胞质基质、液泡和高尔基体则解体消失。成熟原生韧皮部筛分子的原生质组分分布在细胞边缘，由质膜、线粒体、小的滑面内质网堆叠及具拟晶体的Ｐ型质体组成。随着邻近后生韧皮部筛分子分化的进行，成熟原生韧皮部筛分子的原生质组分逐渐退化，最后消失。 When differentiated zephyr virions of Zeamays L. leaf veins begin to differentiate, long rough endoplasmic reticulum and thickening of the cell wall first appear, followed by the appearance of their characteristic pseudocrystal inclusions in the plastid. As differentiation progresses, the long, rough endoplasmic reticulum is transformed into shorter forms, eventually gathering into smaller stacks and losing their ribosomes. Degeneration of the nucleus, but often maintained to the late maturity, when the nucleus from the double nuclear membrane or some parts of the inner nuclear membrane coating, containing electron dense dense amorphous chromatin mass. The bilayer nuclear membrane then ruptures and becomes discontinuous. When the nucleus begins to degenerate, peritubular cavity enlarges locally. Some enlarged peritubular outer nuclear membrane rupture, and accompanied by the adjacent part of the cytoplasm disintegration. In the process of nuclear degeneration, besides the endoplasmic reticulum, the structure of plastid and mitochondria also changed, while the ribosome, cytoplasmic matrix, vacuole and Golgi disintegrated and disappeared. The protoplasmic components of the mature native phloem sieve are distributed at the cell periphery and consist of the plasma membrane, mitochondria, small slide-endoplasmic reticulum stack, and pseudo-crystallized P-type plastids. With the molecular differentiation of adjacent phloem phloem, the protoplasm components of the mature native phloem sieve gradually degenerated and finally disappeared.