Anatomical Studies on the Vascular System of Rice Seedling : I. Network of protoxylem and protophloem

Abstract

Rice seeds coated with calcium peroxide can emerge under anaerobic condition such as in the soil of submerged paddy field. The seedling elongates the coleoptile first and then develops a seminal root and foliage leaves. The same feature of elongation was observed with a hulled rice seedling that grew in the agar medium (Fig. 1). In this paper, we report the anatomy of vascular bundle network distributed in the various organs at the early stage of emergence when mesocotyl achieved to grow. 1. On the transection of mesocotyl, there are two central cylinders, distinguished to large and small ones, and the former is located at the central part and the latter with the scutellum is at the dorsal side. The small central cylinder is a collateral vascular bundle composed of protoxylem and protophloem. In the vascular system of large central cylinder, we observed that both the vessels of protoxylem and the sieve tubes of protophloem lost their cytoplasmic content and these cells changed to pipe-like structure, whereas the cells of metaxylem and metaphloem were not matured enough to lack their content (Fig. 2). The observation indicates that only active conductive tissue consists of protoxylem and protophloem. Protoxylem and protophloem failed to acquire a characteristic arrangement as collateral vascular bundle, and scattered randomly, and this fact has not been reported hitherto. 2. Therefore, we traced the orientation of the protoxylem and protophloem strands in the various organs, mainly mesocotyl (Figs. 5-16), and thus obtained a schematic figure of the whole structure of network as shown in Fig. 3. The protoxylems and protophloems of radial vascular bundle in the seminal root were gathered in a block at the node with their characteristic orientation. Just above the gathering point, protophloems were separated into two directions, one on the dorsal side and the other on the ventral side, while protoxylems remained unseparated. The protoxylems and protophloems were distributed independently in the large central cylinder of mesocotyl. At the coleoptile node, the protoxylems and protophloems ran separately in several directions and then formed collateral bundles which were not found in the lower part of large central cylinder. One of them ran in the reverse direction and then into the scutellum, while the others formed coleoptile and foliage leaf bundles. 3. From these observations, the authors conclude that the large central cylinder of mesocotyl is a transitional tissue system before the radial vascular bundle of seminal root shifts to the collateral vascular bundle of the stem and leaf. Thus, the mesocotyl is an organ that functions as hypocotyl of dicotyledon.