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

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Abstract
The stele of the rice crown root, has 3 kinds of vessels, late metaxylem vessels (MXVII), early metaxylem vessels (MXVI)and protoxylem vessels (PX), as well as 2 kinds of sieve tubes, metaphloem sieve tubes (MP) and protophloem sieve tubes (PP). Observations were 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 constitute the tip of the stele excluding the pericycle, followed by the differentiation 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. In both cases, differentiation proceeded centrifugally. The differentiation of vessels and sieve tubes generally proceeds from the root tip in the order MXVII, MP, MXVI, and, almost at the same time, PX and PP. The observations suggest that there may be a very intimate correlation 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. The joining of the 2 PP accompanied by the shift of the MXVII may also be an example of that correlation. The differentiation of all vessels and sieve tubes proceeds acropetally with continuity. Detailed observation revealed that the formative cells of vessels and sieve tubes do not always form distinct rows longitudinally. 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) by the 2 successive oblique cell divisions. Other kinds of vessels and sieve tubes do not have cells which correspond with the PPMC.

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