On the Numbers of Vessels and Sieve Tubes in Crown Roots of Rice Plants

Abstract

The numbers of late metaxylem vesstls and protophloem sieve tubes were observed at the root tips as well at the mature regions of rice crown roots. Protophloem sieve tubes were selected, because their number usually represents those of other conductive elements of the root, i.e. protoxylem vessels, early metaxylem vessels and metaphloem sieve tubes, respectively. 1. In the case of lower roots, the numbers of vessels and sieve tubes increase towards the higher shoot units, reaching their highest values nearby the X-th shoot unit and decreasing above it. On the other hand, a reverse tendency was found in the case of upper roots. 2. In any root, the numbers of vessels and sieve tubes decrease acropetally. 3. The observation at the mature regions of the all roots revealed intimate correlations between numbers of vessels and sieve tubes and each of root diameter, stele diameter and pericycle cell number. However, only upper roots showed such intimate correlations, while lower roots did not. 4. Quite the same close correlations were found at the root tip where vessels and sieve tubes differentiate. 5. The diameter of the stele increases basipetally and sooner or later comes up to the constant value. And that value is in a high correlation with each of the stele diameter at the positions where vessels and sieve tubes differentiate. From observations mentioned above, it is thought that the numbers of vessels and sieve tubes may primarily be attributable to the dimensions of the part where these tissues differentiate, and that the later development of each tissue of a root may, in general, takes place in keeping pace with one another. It is furthermore suggested that the development and function of lower roots may not always be the same as those of upper roots.