Studies on Morphogenesis in Rice Plant : XII. Ultrastructure of the phloem in the crown root

[摘要] The phloem of rapidly elongating crown roots of rice plants, cultured in water or paddy, were observed by an electron microscope. The intact crown roots were fixed as follows, 1) glutaraldehyde and post osmic acid, 2) ice-cold acrolein-glutaraldehyde and post osmic acid after freezing in aqueous nitrogen, 3) freeze-substutution and post osmic acid. The ultra-thin sections were stained with uranium and lead. The sieve elements of protophloem, having P-plastids, differentiate 300 μm and mature 700-800 μm apart from the growing point. Small vacuoles of the immature sieve elements of protophloem do not participate in the degeneration of cytoplasm. On the other hand, the vacuoles of the sieve elements of metaphloem are expanded in the rapidly elongating zone-1 mm-1 cm apart from the growing point, and the destruction of the tonoplast participates in the degeneration of cytoplasm. The fibillar P-protein and the strand of cytoplasm could not be found in both the immature and the mature sieve elements of proto-and metaphloem. The sieve elements of metaphloem mature about 5 cm apart from the root tip, and just soon after, lateral roots appear. Therefore, on the view point of morphgenesis, the root tissues of 5 cm from the root tip are supported by the sieve elements of protophloem, and the sieve elements of metaphloem support the tissue formation of the basal part of the main root and the elongation of the lateral roots. The sieve elements of proto-and metaphloem have plasmodesmata in the walls facing to pericycle cells and fundamental parenchyma cells as well as to companion cells. These plasmodesmata are thought to be a characteristic of the sieve elements functioning in both sink and long distance transport. The root tissues from 0.5 mm to 2 cm from the growing point are rich in mitochondria, Golgi apparatus and rough endoplasmic reticulum. These cell activities have to be studied in relation to not only morphogenesis, but ion absorption and biochemical synthesis.

[学科分类] 农业科学（综合）