Climbing from the pedicel up into the rachilla, the vascular bundle of the rice spikelet swells in bowl-shape and subsequently piller-shape, branching off the bundles of spikelet organs (Fig. 1 and 3). In the swelling bundle of the rachilla, ramificated xylem and phloem are thrown in confusion, in which tracheids, sieve elements, parenchyma cells and thick-walled cells elaborate a formidable mosaic structure(Figs. 2, 4, 5, 6, 7, 9 and 10). It may be supposed on this structure that, by metabolic activity of the parenchyma cells, water in the sieve elements is extracted out and exuded actively into the tracheids in order to be transpired from the lemma and palea, so that the numerous sieve elements with reduced turgor pressure pump up the sieve current from the pedicel and pump it out toward the dorsal bundle of the ovary. It the dorsal bundle, large xylem sits on the main passway of assimilates from phloem via nucellar projection into endodermis (Fig. 12 and 13), but barium chloride absorbed by the roots does not go up via the vessels of this xylem in normal outdoor condition. This structure might present us with a suggestion that the vessels function in normal condition as drainpipes in order to control the turgor pressure of the ovary and the sieve elements (Fig. 14).
In order to clarify the roles of EF (epidermis-cortical fiber compound) and P (fundamental parenchyma) in breaking resistance, several dynamic characteristics of the fourth internode from the top of rice culm, var. Koshihikari, were studied at the time of harvesting. Breaking strength and deflection of PEF (whole internode tissues) and P, obtained by shaving off the EF from PEF, were measured with aid of the stalk bending hardness tester (type EO-III). Tensile strength, compressive strength and shearing strength were measured by traditional methods used in the material testing. These data are shown in Table 1. Some calculations with these data verify that EF/P ratio in YOUNG's modulus, tensile strength and compressive strength are 113, 63 and 19, respectively. Besides, the tensile strength of EF in rice internode was ascertained to be almost the same to that of the xylem tissue of boxwood (Buxes microphylla var. suffruticosa Makino). Moreover, about 89% of bending stress (tensile and compressive stress) is supported with EF, and about 78% of shearing stress is supported with P in the bended internode. Therefore, with regard of bending stress, the internode of rice culm can be considered dynamically to be a pipe composed of thin EF as illustrated in Fig. 1. The value of maximum stress M can be obtained by the formula (6), and d2t [(diameter of internode)2×(thickness of EF)] can be considered to be a very important factor in breaking resistance. On the other hand, the main role of P in breaking resistance is to prevent sectional deformation and to maintain cylindrical form in the bended internode. Breaking strength of the fourth internodes with or without leaf sheath was investigated in the ripening period as shown in Fig. 2. The breaking strength had a peak soon after heading and a minimum at 25-30 days after heading. It may be thought that the slight increase in breaking strength at the time of harvesting was caused by reaccumulation of starch in the internode.
The ultrastructure of small vascular bundles of the 8th leaf blades in rice seedlings was examined with the electron microscope. 1. In the parenchyma sheath cells the chloroplasts have a centrifugal position, whereas the mitochondria have a centripetal position. The phloem is bounded by a single layer of thick-walled parenchyma cells, which differentiate into the mestome sheath cells in the upper leaves. The thick-walled parenchyma cells have the plastids with starch granules, mitochondria and dictyosomes. The suberized lamellae are not detected in the walls of these sheath cells. 2. The protophloem sieve elements and companion cells locatcd abaxially within the vascular bundles show an apparent degeneration in the expanded leaf, but the degeneraion is not observed in the elongating zone of the folded leaf. Thus, the protophloem may degencrate during the leaf emergence and ceases to function after leaf expansion. 3. Late-formcd metaphloem sieve elements are narrow and thickwalled, but the associated companion cells are wider than the sieve elements. The metaphloem sieve elements have the P-type plastids and a small amount of mitochondria and endoplasmic reticulum. The rnetaphloem companion cells contain many mitochondria and endoplasmic reticulum, and often contain osmiophilic globules. Ultrastructural features of the metaphloem parenchyma cells are the same as the companion cells. 4. The distribution of plasmodesmata in transections of the small vascular bundles has been determined. The plasmodesmata occur in the outer and inner tangential walls of the parenchyma sheath cells. The late-formed metaphloem companion cells are connected by numerous plasmodesmata with adjacent parenchyma cells of the metaphloem and metaxylem, and also with the thick-walled parenchyma cells respectively. The companion cell and the sieve element are connected by plasmodcsmata which are branched on the companion cell side. The data support the view that photosynthate moves through a symplastic pathway from mesophyll to the metaphloem sievc elements. The following pathways are suggested.[table]
In the swelling piller-shape part of rachilla vascular bundle of rice spikelet, ramificated xylem, phloem and unlignified thick-walled parenchyma cells elabolate a mosaic structure (Fig. 1 and 2). There can be observed no transfer cell, but sieve elements are surrounded with a number of large phloem parenchyma cells including abundant mitochondria (Fig. 1, 2 and 6), and the thick-walled parenchyma cells are rich in simple pits and smooth endoplasmic reticulum (Fig. 3 and 6) connecting with cortical sclerenchyma cells (Fig. 2). For about 25 days after flowering, starch grains are observed only in the cortical sclerenchyma cells and the thick-walled parenchyma cells of rachilla bundle (Fig. 6), and subsquently disapear. By tracing barium chloride absorbed through the roots, no transpiration stream can be observed in the piller-shape part of the rachilla bundle. It is supposed, that the phloem parenchyma cells provide the sieve elements with the energy necessary for phloem active transport, that the turgor pressure of the sieve elements is controlled by putting in and out the water and solutes of sieve elements, and that the tracheids function as drainpipes of water.
The ultrastructure of vascular bundles and fundamental parenchyma (parenchyma in fundamental tissue system) in the 8th leaf sheath of rice were examined with a light and an electron microscopes in reference to possible pathways for photosynthate between phloem and fundamental parenchyma. 1. In the phloem of small bundles, the sieve element-companion cell complexes located in the middle and adaxial side of phloem remained without degeneration after sheath elongation (Fig. 1). However, the degeneration of phloem in large bundles is earlier than that in small bundles, and only several sieve element-companion cell complexes that abut on xylem remained without degeneration in the second leaf sheath from the uppermost fully-expanded leaf (Fig. 5). 2. In the boundary of xylem and phloem in small and large bundles, the plasmodesmatal connections were found in each interface between sieve element-companion cell complexes and xylem parenchyma cells, and between parenchyma cells of xylem and phloem (Table 1, Fig. 1). These connections may play a role in transfer pathways to phloem for solutes absorbed from transpiration stream by xylem parenchyma cells. 3. Cell walls in phloem of large and small bundles were densely stained purple with toluidine blue O (Figs. 3 and 4) and the plasmodesmatal connections were rare in the walls between sieve element-companion cell complexs and phloem parenchyma cells (Table 1). The phloem parenchyma cells contained many mitochondria with well-developed cristae and remained without degeneration after sheath elongation (Figs. 1, 2, 6 and 7). These data support the view that sucrose moves in the apoplast between sieve element-companion cell complexes and phloem parenchyma cells. 4. Suberized lamellae occur in all walls of the mestome sheath cells of small and large bundles (Figs. 1, 7, 8 and 9). Aggregates of plasmodesmata were observed in the walls between phloem parenchyma cells and mestome sheath cells (Figs. 1 and 7), between mestome sheath cells and fundamental parenchyma cells (Figs. 8 and 9), and also between fundamental parenchyma cells (Figs. 10 and 13). Judging from these observations, it appears that sucrose moves in the symplast between phloem parenchyma cells and fundamental parenchyma cells. 5, Different types of plastids were contained in the fundamental parenchyma cells of leaf sheath. The plastids in the top of sheath were similar in structure to the chloroplast (Fig.11), whereas those in the base of sheath were the typical amyloplasts having large starch grains and a few thylakoids (Fig. 13). The plastids in the middle of sheath showed an intermediate structure of chloroplast and amyloplast (Fig. 12).
In the leaf blades, sheaths and internodes of rice plants, the large and small vascular bundles are arranged longitudinally. When the longitudinal vascular bundles of the leaf blades are traced backward into the culm, the large vascular bundles extend downward through the leaf sheath and two internodes. The small vascular bundles, which are located in the middle between large vascular bundles of leaf blade, extend downward through the leaf sheath and one internode, and the other small vascular bundles end blindly at the base of leaf blade. In the leaf blades and sheaths, the longitudinal vascular bundles are laterally interconnected by the transverse veins. The transverse veins have the most simple composition of the vascular element, that is, a sieve tube, a vessel, and only two files of vascular parenchyma cells. In the leaf blades at lower levels of shoot, the small vascular bundle is surrounded by a single layer of parenchymatous sheath cells with dense chloroplasts. However, in the leaf blades at higher levels of shoot, the small vascular bundle is surrounded by two layers of sheath cells. The outer layer is consisted of parenchymatous sheath cells with small numbers of chloroplasts, and the inner layer is consisted of mestome sheath cells with thickened walls. The mestome sheath of the large vascular bundle is more developed than that of the small vascular bundle. The vascular bundles of the leaf sheaths and internodes have more companion cells and larger sieve tubes and vessels than those of the leaf blades. On this histological observations, the functional significance of the vascular bundles in rice plants is discussed.
As the result of an observation on the elongating process of leaf blade, leaf sheath, panicle and internode in all over the growing period of rice plant, a regularity of development was recognized among those organs, and a problem was proposed that the regularity is due to the constitution of vascular bundles and its ripenning process in the culm. By the way, the meristem concerning with the culm formation were studied by histological methods, with special reference to the intercalary meristem of internode.
The structure and the cellulose microfibril orientation of cell walls of cortical fibre cells, parenchymatous cells and vessels were observed with the technique of microscopic chemistry and polarized microscopy. The cell wall of the cortical fibre is composed of three layers, and the vessel has two kinds, dominant and recessive, of microfibril orientation, as illustrated in Fig. 1 (p. 603). It is important that, in all cases, the cellulose microfibrils cross each other at the angle of about 84°or about 58°Notes. Photos. 1-10. Cortical fibre cell. Photo. 1. Transverse section. The outer layer (S1) is formed first, and followed by the middle layer (S2) and the inner layer (S3). Photos. 1A', 1B' and 1C' are photographed between crossed nicols. PW Primary wall. Photos. 2 and 3 are the transverse sections of the middle layer and the outer layer, respectively, swollen with cuprammonium. The middle layer shows several lamellae. Photos. 4 and 5. The cortical fibre cells swollen with sulfuric acid, show the helices of the outer layer. Photos. 6 and 6'. Surface view of a single wall of the outer layer. Photo. 6 is the wall stainde with iodine, and 6' is photographed between crossed nicols. The axes of pits and striations run in the same direction. Photo. 7. Surface view of a single wall of the middle layer. Photo. 8. Surface view of a single wall of the inner layer. ST Station. Photo. 9. The inner layer's helices of the cortical fibre swollen strongly with cuprammonium. Photos. 11-15. Parenchymatous cell. Photo. 11. The transverse section of a cell wall, swollen with sulfuric acid and stained with iodine, shows many lamellae. Photo. 11 is photographed through polarizer. The dichroism of iodine can be recognized. Photo. 11' is photographed between crossed nicols. Photos. 13 and 14. Surface view of a vertical single wall bearing two sets of striations crossing each other at the angle of about 84°. Photo. 15. Surface view of a horizontal single wall having striations running at random. Photos. 16 and 17. Annular vessel. Photos. 16' and 17' are photographed between crossed nicols. Microfibrils run horizontally in the hollow rings. Photos. 18 and 19. Pitted vessel. Photos. 18A and 19A are focused on the basal plane, 18B and 19B at the top. The direction of pit axes shows that microfibrils run downward along an S helix. Photo. 20. The cell wall of a vessel, swollen with sulfuric acid, shows several lamellae. Photo. 21. Surface view of a single wall of the vessel. Two sets of striations cross each other at the angle of about 84°in 21 A and B, and at the angle of 58°in 21C and D.
Ultrastructure of the large vascular bundles in expanded and folded leaves of rice plants were observed by an electron microscope, and the results were compared with those of the small vascular bundles previously reported. 1. The large bundles have more developed mestome sheath than the small bundles. The suberized lamellae occur in the outer and inner tangential walls of the mestome sheath cells, but are absent in the middle portion of the radial walls. The lamella occurs between primary and secondary walls of the mestome sheath cells. 2. The protophloem sieve elements and companion cells located abaxially within the large bundles, mature in the elongating zone of the folded leat and begin to degenerate before emergence of the leaf. So far as the same portion of the leaf is concerned, the maturation and degeneration of the protophloem in large bundles are earlier than those in the small bundles. 3. In the border region between phloem and xylem of the large bundles, the parenchyma cells of metaphloem and metaxylem, and the cells of mestome sheath are interconnected by numerous plasmodesmata. According to this observations, the following symplastic pathways for photosynthate are suggested. [figure] 4. The large bundles have many metaxylem parenchyma cells, in which abundant mitochondria and endoplasmic reticulum are contained. The function of the metaxylem parenchyma cells in absorption and transfer of solutes is discussed.
The process of regeneration was observed histologically, after the tip of seedling root, 2--3cm long, of Zea Mays had been decapitated or incided longitudinally. 1. Decapitation case --- The observation of the root tip which had been decapitated in the plane at different distances from the growing point, revealed that the tissue between the growing point and the part 400μ deep toward the base from the point regenerates a normal root apex (table 1). It was then confirmed that the decapitation brings about a callus formation and the pattern of the root apex is regenerated immediatly and centripetally (fig. 3). After the completion of a root apex, the new root tip starts elongating. 2. Incision case--With a single or a cross incision of a root apex, two or four normal roots were regenerated (fig. 1). The histological observation on incisions at different depths from the growing point, revealed that the callus formation is limmited within the wounded zone 400μ away from the growing point (table 2). The callus fomation in this case spreads basipetally from the growing point, and even late metaxylar vessels produce their derivatives. The position of a new growing point can be recognized at the very early stage of regeneration (fig. 4, 5), but the stele in the basal portion begins to be reconstructed befote the recompletion of the root apex (compare fig. 6A with fig. 4B). The observation on the transections indicated that the kind of the tissue, into which the derivatives of late metaxylar vessels mature, depends upon the position of derivatives in the horizontal plane (fig. 6). All the late metaxylar vessels differentiated soon after the apical meristem has been regenerated, are always connected with some of the old vessel elements (fig. 7). It may thus be concluded that not only basipetal but also acropetal as well as horizontal functions among cells or tissues shoud be recognized in the part of the root tip 400μ deep from the growing point toward the base, and these three functions work together to maintain the normal organization and differentiation. Fig. 1. Normal four roots regenerated after a longitudinal cross inclsion of the root apex. 7 days after the operation. - Fig. 2. External view of root tips 2 days after the incision. Incising depths from the growing point are, from the right, 50-100μ, 100-150μ, 200-250μ, 300-400μ, 400-500μ, and 600-700μ, - Fig. 3. Longitudinal sections of root tips decapitated at the plane 100-150μ away from the growing point. LX, late metaxylar vessel ; DG, dermatogen; CG, calyptrogen. A: 1 day after the operation, callus parenchymatous cells being derived all over the wounded plane. B: 2 days after the operation. C: 4 days after the operation, the pattern of root apex having been almost reformed. ×80. - Fig. 4. Longitudinal sections of the halves of root tips incised longitudinally. The wounded plane is in the right part of each photograph. A : 7 hours, B : 1 day, C : 2 days after the operation. GP, growing point. Note that wound callus formation spreads basipetally from the old growing point and the position of a new growing point has already become recognizable 1 day after the operation. Fig. 5. Transections of a root tip 1 day after incision. A : 200-250μ, B: 150-200μ, C : 100μ, each away from the growing point at the time when the root tip was operated. D: left, 90μ away from the growing point, and right, close to the growing point. Cell division can be recognized more frequently in the apical part than in the basal, and all the late metaxylar vessels have produced their derivatives. ×140. - Fig. 6. Transections of the root tips at the portion of 100-150μ away from the growing point at the time when the root tips were incised. The wounded, plane is in the lower part of each photograph. A: 2 days, B: 4 days, C: 7 days after the operation. EN, endodermis; PX, protoxylar vessel. [the rest omitted]
