On the Differentiation of Vessels and Sieve Tubes at the Root Tips of Rice Plants

Abstract

It is known that in the stele of the rice crown root, there are three kinds of vessels-late metaxylem vessels (MXVII), early metaxylem vessels (MXVI) and protoxylem vessels (PX) -as well as two kinds of sieve tubes, metaphloem sieve tubes (MP) and protophloem sieve tubes (PP). Our observation was carried out on the process of the differentiation of those vessels and sieve tubes at the crown root tip.
Of the vessels, the MXVII differentiate earliest from the “large cells” which con stitute the tip of the stele excluding the pericycle (Figs. 1 and 2), followed by the dif ferentiation of the MXVI and lastly the differentiation of the PX takes place. Of the sieve tubes, the differentiation of the PP follows that of the MP. Thus, in both cases, the differentiation was found to proceed centrifugally. On the whole, the differentia tion of vessels and sieve tubes proceeds from the root tip in the order MXVII, MP, MXVI, and, finally, almost at the same time, PX and PP.
The observation mentioned above suggests that there may be a very intimate corre lation between the differentiation of each vessel and that of each sieve tube; e.g., a close correlation was found between the differentiation of the MXVI and that of the PX (Fig.14). The “joining” of the two PP (such a phenomenon as that in Fig.13 D→ A) accompanied by the “shift” of the MXVII (such as that in Fig.6 A→ D) may also be an example of that correlation.
The differentiation of all vessels and sieve tubes proceeds acropetally with con tinuity. Detailed observation, however, revealed that the formative cells of vessels and sieve tubes do not always form distinct rows longitudinally (Figs. 4, 5, 6 and 13). Consequently, it can be thought that the differentiation of vessels and sieve tubes in the root proceeds under influences exerted both from the root apex and the basal part.
The PP differentiate from the “protophloem mother cell” (PPMC) (Fig.8) by the two successive oblique cell divisions (Figs. 9, 10, 11 and 12). Other kinds of vessels and sieve tubes, however, do not have cells which correspond with the PPMC.