Mechanism on the Development of Deformed Spikelets in Oryza Sativa L. : II. The dinstinction between the main vascular bundle of spikelet axis and that of floret one in the normal spikelet

In the previous paper, a hypothesis (Model N), interpreting the morphological nature of the normal spikelet in rice plant, was proposed. And it was postulated that the main vascular bundle in the axis of the normal spikelet consists of two kinds; that of spikelet axis (rachilla) and that of floret one, and their connective point presents in the axis between upper empty glume and palea. In this paper, the course of vascular bundle of the normal spikelet is traced with serial-microtome sections. The main vascular bundle extends to lateral direction in the level just above the abcission layer (Plate 1-1∼3) and forms a ring (Plate 2-1), that is, a vascular cylinder. As shown in Plate 1, it is clear at higher level that the main vascular bundle has a large gap (arrow symbols in Plate 1) between the both bases of midrib trace of lemma and palea (a and o in Plate 1). And the main vascular bundle forms a solid column from the gap to the base of pistil. To research the detail of the vascular course around the gap is an important problem. The trace of midrib of lemma (a in Plate 2-2) detaches itself from the vascular cylinder at the ventral side, forming an incomplete gap on this cylinder (Plate 2-1∼3). The half tissue of the vascular cylinder containing left and right parts of the leaf trace "a", increases in the thickness at the ventral side and two lateral leaf traces diverge into the latero-ventral position from the both parts (b and c in Plate 2-3). These traces connect with each lateral veins of lemma. Subsequently the both parts of the vascular cylinder join together inside of the cylinder and synchronously detach themselves from the dorsal arc of the cylinder (Plate 2-4∼5). In this way, the main vascular bundle of solid column type is composed at the ventral side, namely at the axil of lemma (vfa in Plate 2-6, 7). And this main vascular bundle diverges the traces into the flower organs; palea, lodicules, stamens and pistil (see Plate 1). However, it does not always occur in palea as follows. The dorsal arc of the vascular cylinder divides into four fragments after the divergence of a trace (c2 in Plate 2-5, 7), forming a gap, into upper empty glume. Two fragments of outer side (m and n) supply a trace to each marginal veins of lemma. Two remnants (w and z) join again each other as shown in Plate 2-8, and connect with the midrib of palea (o in Plate 2-11). Therefore, the midrib of palea, not producing any gaps, is classified into two kinds according to its origin (one; "o" in Plate 1, the other; "o" in Plate 2). The main vascular connection between "cylinder" and "solid column" is observed at the nordal region, where axillary axis (floret axis) branches out from main axis (spikelet axis), as shown in other grass spikelets, for example, Triticum and Bromus. The vascular system between the main vascular bundle of solid column type and traces of the flower organs resembles to that of floret in other grass spikelets, except that the traces of palea does not always diverges from the main vascular bundle of solid column type. Moreover it seems that these vascular courses in the normal spikelet are quite alike an unilacunar type as usually seen in many dicots. The conclusion obtained from these comparative anatomical aspects is that the main vascular bundle of solid column type at axil of lemma is considered to be a branch trace in connection with the vascular cylinder. In other words, the vascular cylinder and its branch trace are correspond to the main vascular bundle of spikelet axis and that of floret one, respectively. It can be said that our conclusion does not contradict with the hypothesis (Matuba, 1971) shown previously. Such anatomical analysis leads us to a conclusion that the axis in the normal spikelet consists of two kinds; spikelet axis and floret one, in spite of the external appearance as same as single axis. [the rest omitted]