Intra and Inter-Cellular Changes in Constitution during the Development of Laticiferous System in Garlic Scape

Systematic investigations, mainly based on electron microscopy, have been conducted on constitutional changes in the laticifer and its adjoining parenchyma during the development of laticiferous system in garlic scape. The laticiferous system of the scape consists of several layers of articulated unbranched latieifers. About half of them are situated 2–3 cell layers below the epidermis, and the rest scattered throughout the cortex (Fig. 23). Latieifer differentiation starts with a thinning out and vacuolation of the dense protoplasm in the latieifer initials (Fig.2), which is followed by gradual degeneration of nuclei, plastids, endoplasmic reticulum, and dictyosomes; and by a sharp diminution of free ribosomes (Figs.3, 4). Remanent and defective forms of some organelles can still be found in the laticifer at the later stage. In spite of these drawbacks, the differentiating laticifer appears to function actively. Its protoplasm is delimited by a distinct plasmalemma (Figs. 3, 4). Its wall is interspersed with pits inclose spacings, to which most plasmodesmata are confined (Figs. 8, 24). The cell interior is packed with vesicles and mitoehondria (Figs. 4, 11, 15). Structurally, the laticifer seems well adapted to material exchange with the adjoining parenchyma. During the sprouting stage of the scape, the laticifer initials enlarge itself or fuse with each other by lateral wall dissolution to extend the diameter; at the same time, the laticifer elongates at an increasingly rapid rate. As a final result, the laticifer can attain 30–50 times the length and 2–3 times. the diameter of the adjoining parenchyma. The electron-dense material which protrudes into the laticifer initial from the parenchyma may be of lysosomal nature and probably concerned with wall dissolution and intracellular lytic processes in latieifer formation (Figs. 5, 7, 10). An excised garlic scape is employed in the observation of mature laticifers, which is always full of sap and is quite turgid. Once the scape is cut open, sap exudes almost exclusively from the cut end of the laticifers at the periphery, which lasts only some seconds. However, if the scape is left aside for a few days, exudation will again take place at the fresh cut end. Unlike the milky juice of many latex plants, the sap exuded from the garlic scape is watery and slightly turbid. The organic solute content is mainly made up of simple sugar and amino acids. It also contains a small amount of proteins and even protoplasmic fragments. Besides, it is worthy to note that decrease in organic solutes in the exudation is closely connected with the degree of exhaustion of cell contents from the withering scape, which is, as has already been shownm the sole agent of supplying materials required for the formation of apical cloves. All the above facts seem to indicate that there exists a loading and unloading process in the latieifer. Our electron micrographs (Figs. 16, 22) give evidence that vesicular transport through plasmodesmata in the pit field is capable of performing such a process: from the parenchyma to the laticifer in loading and from the latter to the former in unloading. The possible role of the laticifers in garlie scape could be a temporary storage of cell contents released successively from the deteriorating parenchyma. The sap content in the laticifer is in full turgidity as a result of loading, and can be readily drawn by unloading if so required. Transcellular cytosis is a term tentatively given by us to designate intercellular transport of sap, solutes, and macromolecular particles in small vesicles, which are formed and packed in one celt, traverse through plasmodesmata and merge into the other; whereas endo and exoeytosis refer to vesicular transport in a single cell only and to its moving in and out of the cell primarily through the plasma membrane, which also takes active part in the formation and dissolution of the vesicle and in the enclosure and release of its content. Transcellular cytosis was first observed by us in the withering parenchyma of an excised garlic scape; and, in the present case, between the latieifer and parenehyma, both being active functionally. As compared with the early notion that intercellular material transport is primarily carried out by secretion and reabsorption of highly degraded products through plasmalemma, transcellular eytosis appears to be a far more efficient means of translocating prefabricated assortment and well packed cargo from one cell into the other.     

浙贝母鳞茎衰退过程的解剖学研究初报

采用徒手制片、石蜡切片和电子显微镜技术,系统而深入地研究了浙贝母（ＦｒｉｔｉｌｌａｒｉａｔｈｕｎｂｅｒｇｉｉＢａｋｅｒ）鳞茎衰退过程中的形态、组织结构和细胞内部的变化情况．看到了大分子物质进行运输时采用集装囊泡形式的可能性。进一步论证了胞间连丝作为细胞间原生质通道的观点．     

The Structural Changes During the Degeneration Process of Antipodal Complex and Its Function to Endosperm Formation in Wheat Caryopsis

During the early developmental stage of wheat caryopsis the antipodal complex (composed of 20 or more cells) located on the chalazal part of embryo sac gradually turns to degeneration and degradation from its outer part to the innermost, undergoing apparent structural changes of protoplasm. The senescent tissue (antipodals) exports its cell contents continually to support the proliferation and enlargement of the adjacent free-nuclear endosperm and accommodate the dual function of both material transport and nurture supply. The lacking of callose deposition on the boundary wails between antipodals and endosperm is much benefit to the solute transport, but not all cell contents in antipodals undergo thorough degradation until exporting, at least, part of the protoplasm only undergoes limited structural disintegration. The disassembled protoplasmic constituents actively migrate through symplast route in the form of macromolecule. This shows another mode of material transport in feeding endosperm. The occurrence of wide cytoplasmic channel in part of boundary wal ls berween antipodals and endosperm shows a special structural transformation of intercellular connection. Therefore, disassembled nuclear materials, cisternae of endoplasmic reticulum and plastids, mitochondria, etc. could migrate from antipodals into the developing endosperm. It is deduced that this mode of material transport may play an important role in supporting rapid proliferation and enlargement of free-nuclear endosperm in the developing caryopsis.     

The Endodermoid in Garlic Scape: Its Fine Structure and Physiological Function

In garlic scape there is a distinct boundary layer of cells between the cortex and stele. Its fine structure and possible ruction seem to agree with the endodermoid first defined by Esau[7], hence the frame. Lightand electron-microscopic examination and cytochemical test have revealed that this particular laryer is probably responsible for the withdrawal of cellular contents of parenchyma to the peripheral vascular bundles, during a long period of storage the excised withering scape would thoroughly exhaust itself to give rise to the new apical cloves. As already shown, the laticiferous tubes scattered throughout the cortex are always turgid, and their sap is rich of nutrients in variable proportion[3]. Possibility of mobilizing these nutrients by the aid of endodermoid to join in phloem transport also has been discussed.     

Ultrastructural Study on the Senescent Process of Scales in Fritillaria thunbergii Miq.

The senescent process of scales in Fritillaria thunbergii Miq. was observed by means of light and transmission electron microscopy. Several layers of parenchymal cells near the adaxial cortex degraded at the outset, forming a clear broken cell zone. The degradation of cell protoplasm proceeded actively and orderly. Dictyosomes and endoplasmic reticulums produced many vesicles which were of priority importance during the process of protoplasmic degradation and intercellular transport of the degraded products. The abundant plasmodesmata between cells provided an efficient channel for the intercellular transport.     

浙贝母鳞片衰退过程的超微结构研究

通过光镜和电镜手段观察了浙贝母（Ｆｒｉｔｉｌｌａｒｉａ ｔｈｕｎｂｅｒｇｉｉＭｉｑ．）鳞片的衰退过程．开始时,近轴面表皮附近的几层薄壁细胞首先瓦解,形成一条清晰的破碎细胞带．细胞内含物的降解过程是主动有序的．高尔基体和内质网产生许多囊泡,囊泡在细胞内含物的降解和降解产物的运输过程中起着重要的作用．细胞间丰富的胞间连丝是胞间物质运输的良好通道     

The Structural Changes of the Basal Region of Wheat Proembryo in Relation to Nurture Absorption

There are some cellular fail and degeneration in the parietal area of the basal region of developing wheat proembryo. Electron microscopic studies reveal that the envelopment of peripheral wall to the proembryo is partly ruptured in this area and the disassembled protoplasm of the degenerated cells mixes with the disintegrated constituents of adjacent endosperm cells. Hence, in the limited area a direct communication between the inner surviving proembryo cells and the surrounding medium is established. A number of ectodesma-like plasmodesmata and open channels appear at the boundary wall, various nutrients may enter the proembryo via symplastic pathway or by endocytosis. The surrounding macromolecules (disassembled nuclei, mitochondria, cytoplasmic granules and vesicles packed with fibrils) appear to traverse across the wall continually, and it seems that this is'an important mode of nurture translocation. Also, within the proembryo some of the densely distributed plasmodesmata undergo modification and become fully opened for macromolect, les traversing, which is in favor of re-distribution of cell contents amongst proembryo cells. Presumably, the structural changes occurred in the basal region is a special kind of differentiation which results in function of this local area as apparatus of nurture absorption. Evidently, it would enhance the incorporation of external materials into the proembryo, and then the normal proliferation, development and differentiation of proembryo cells would be ensured.     

离体蒜苔贮存中薄壁细胞超微结构的变化

离体蒜苔贮存中薄壁细胞逐渐衰老,原生质表现出有序的降解和胞间转移。由细胞内膜产生的自体吞噬泡将细胞器消化。降解的原生质组分通过共质体和质外体两种途径转移,并最终运输至顶端作为珠蒜生长的营养。     

Intercellular movement of protoplasm in vivo in developing endosperm of wheat caryopses

Summary Earlier work in our laboratory indicated that protoplasmic constituents can migrate from one cell to another in certain tissues of higher plants. Further investigations have been conducted using garlic bulbs and wheat nucellus for microscopic observation of intercellular protoplasmic movement in vivo. These gave preliminary indications of the dynamic characteristics of migrating nuclei and cytoplasm. The present paper gives recent results providing new evidence for intercellular protoplasmic movement that is neither hindered by the presence of cell wall nor the narrowness of channels of intercellular connection. By careful manipulation, intact endosperm sacs could be taken from developing caryopses (6–8 days after fertilization) without apparent injury to constituent cells. Shortly after the living specimen is mounted on the microscopic stage, asynchronous intercellular protoplasmic movement can be observed here and there. It can be seen that protoplasm extrudes in rapid but intermittent movements from one cell to the next by vigorous contraction. Although various cell constituents may move together, they can also be quite independent of each other. The moving units, though undergoing violent deformation, resume their normal shape and structure following intercellular migration. Evidently this kind of movement is a naturally occurring and active phenomenon closely related to the physiological state of the tissue. Electron microscopic studies reveal that a limited number of plasmodesmatal channels undergo modification and enlarge to 100–400 nm, through which the protoplasmic constituents pass.     

A Preliminary Study on the Structure and Function of the Vascular Transfer Cells in Garlic Scape

The vascular transfer cells in garlic scape havebeen examined with electron microscope. Their structure, distributive feature and adenosine triphosphatase (ATPase) activity are studied. The mature vascular transfer cells exhibit the characteristic cell wall ingrowths. The cell contents include a large nucleus, dense cytoplasm and various normal organelles. It is notable that there are numerous mitochondria with well developed, cristae. Plasmodesmata are extensively present in the wall, and transfer cells are connected to adjacent cells by them. The senescing transfer cells become more vacuolated and have a large central vacuole and dense parietal cytoplasm. Their wall ingrowths seem to degenerate and finally disappear. The transfer cells show a particular pattern of distribution in the vascular bundle of the garlic scape. Some of them are present between the vessels of xylem and the sieve tubes of phloem. However, more abundant cell wall ingrowths occur on those walls which abut on, or are close to the vessel of xylem. The other transfer cells are located between the sieve tubes and parenehyma cells. The phloem transfer cell which is adjacent to sieve tube has developed from companion cell. All the transfer cells are mainly concerned with the loading and unloading of sieve tubes. And they may play an important role in facilitating intensive material transfer between two independent systems (i.e. the vessels and sieve tubes, the symplast and apoplast). The results of the cytochemical localization of ATPase using a lead precipitation technique exhibit strong enzyme activity on the plasmalemma of the transfer cells. It is suggested that the transfer cells are especially active in solute movement through them to which cellular energy metabolism coupled.     

Plasmodesmatal frequency in relation to short-distance transport and phloem loading in leaves of barley (Hordeum vulgare). Phloem is not loaded directly from the symplast

We investigated the phloem loading pathway in barley, by determining plasmodesmatal frequencies at the electron microscope level for both intermediate and small blade bundles of mature barley leaves. Lucifer yellow was injected intercellularly into bundle sheath, vascular parenchyma, and thin-walled sieve tubes. Passage of this symplastically transported dye was monitored with an epifluorescence microscope under blue light. Low plasmodesmatal frequencies endarch to the bundle sheath cells are relatively low for most interfaces terminating at the thin- and thick-walled sieve tubes within this C₃ species. Lack of connections between vascular parenchyma and sieve tubes, and low frequencies (0.5% plasmodesmata per μm cell wall interface) of connections between vascular parenchyma and companion cells, as well as the very low frequency of pore-plasmodesmatal connections between companion cells and sieve tubes in small bundles (0.2% plasmodesmata per μm cell wall interface), suggest that the companion cell-sieve tube complex is symplastically isolated from other vascular parenchyma cells in small bundles. The degree of cellular connectivity and the potential isolation of the companion cell-sieve tube complex was determined electrophysiologically, using an electrometer coupled to microcapillary electrodes. The less negative cell potential (average –52 mV) from mesophyll to the vascular parenchyma cells contrasted sharply with the more negative potential (–122.5 mV) recorded for the companion cell-thin-walled sieve tube complex. Although intercellular injection of lucifer yellow clearly demonstrated rapid (0.75 μm s^-1) longitudinal and radial transport in the bundle sheath-vascular parenchyma complex, as well as from the bundle sheath through transverse veins to adjacent longitudinal veins, we were neither able to detect nor present unequivocal evidence in support of the symplastic connectivity of the sieve tubes to the vascular parenchyma. Injection of the companion cell-sieve tube complex, did not demonstrate backward connectivity to the bundle sheath. We conclude that the low plasmodesmatal frequencies, coupled with a two-domain electropotential zonation configuration, and the negative transport experiments using lucifer yellow, precludes symplastic phloem loading in barley leaves.     

宁夏枸杞果实韧皮部及其周围细胞超微结构研究

应用透射电镜技术研究了宁夏枸杞果实韧皮部细胞的超微结构变化。结果表明:(1)随着枸杞果实的发育成熟,果实维管组织中的韧皮部筛分子筛域逐渐变宽,筛孔大而多,通过筛孔的物质运输十分活跃;筛分子和伴胞间有胞间连丝联系,伴胞属传递细胞类型,与其相邻韧皮薄壁细胞和果肉薄壁细胞连接处的细胞界面发生质膜内突,整个筛分子/伴胞复合体与韧皮薄壁细胞之间形成共质体隔离,韧皮部糖分的卸载方式主要以质外体途径进行。(2)韧皮薄壁细胞间的胞间连丝较多,而韧皮薄壁细胞与果肉薄壁细胞的胞间连丝相对较少,但果肉薄壁细胞间几乎无胞间连丝;果肉薄壁细胞之间胞间隙较大,细胞壁和质膜内突间形成较大的质外体空间,为质外体的糖分运输创造了条件。(3)筛管、伴胞、韧皮薄壁细胞和果肉薄壁细胞中丰富的囊泡以及活跃的囊泡运输现象,暗示囊泡也参与了果实糖分的运输过程。研究推测,枸杞果实韧皮部同化物的卸载方式以及卸载后的同化物运输主要以质外体途径为主。     

STRUCTURE AND DEVELOPMENT OF SIEVE ELEMENTS IN THE LEAF OF ISOETES MURICATA

Leaf tissue of Isoetes muricata Dur. was fixed in glutaraldehyde and postfixed in osmium tetroxide for electron microscopy. The very young sieve elements can be distinguished from contiguous parenchyma cells by their distinctive plastids and the presence of crystalline and fibrillar proteinaceous material in dilated cisternae of the rough ER. During differentiation, the portions of ER enclosing this proteinaceous substance become smooth surfaced and migrate to the cell wall. Along the way they apparently form multivesicular bodies which then fuse with the plasmalemma, discharging their contents to the outside. At maturity, the sieve element contains an elongate nucleus, which consists of dense chromatin material, and remnants of the nuclear envelope. In addition, the mature sieve element is lined by a plasmalemma and a parietal, anastomosing network of smooth ER. Both plastids and mitochondria are present. P-protein is lacking at all stages of development. Tonoplasts are. not discernible in mature sieve elements. The end walls of mature sieve elements contain either plasmodesmata or sieve pores or both, but only plasmodesmata occur in the lateral walls.     

Transport of Disassembled Protoplasm from Degenerated Nucellus into Embryo Sac and Its Role in Feeding the Proliferating Antipodals in Wheat

The wheat embryo sac is pear-shaped and deeply imbedded in fleshy nucellus of uneven thickness, which, in mm, is enclosed partially by two layers of integument and is in intimate connection with the procambium around the chalazal region (Text fig. 1 ). This connection seems to be the main inlet passage of nutrient in the ovule. Accordingly the nutrient has to pass from cell to cell and to be incorporated in the nucellus before it is fed to the enlarging embryo sac. Though as a whole the nucellus is transitory in existence, establishment of new peripheral layers and decline of inner layers occur at the same time. While cells in the outer layers multiply by mitosis; cells in the intermediate layers begin to exhibit “nuclear extrusion” (an indication of transcellular movement of protoplasm) which becomes more frequent in inner layers (Text Fig. 2); and cells in the innermost layer, embracing the embryo sac, actively undergo disintegration, showing walls in rupture and cellular contents in disassembly and in retreat (Fig. 7,8). A distinguished feature of high activity of ATPase located in extruding nucleus has been observed in chalazal region (Fig. 4) and in degenerating nucellag cells (Fig. 5). The embryo sac is delimited from the nucellus by an incomplete envelop at the mycopylar end, and the envelop is reinforced by successive deposition of wall debris of the diminished nucellar layers (Fig. 9); whereas at the chalazal end the envelop is lacking and the anfipodals can communicate directly with the disintegrating layer of the thickened portion of nucellus. The antipodals grow steadily as the embryo approaches maturity and the number of cells can be increased 7–8 times(from 3 to 20 or more). Conceivably, proliferation of the antipodals is sustained at the expense of the disintegration of nucellar tissue. The present investigation has confirmed our previous statement that transport of disassembled protoplasm is involved in the feeding of antipodals by nucellus. Some electron micrographs are chosen to reveal details of this particular process. Some findings of special interest are listed below: 1. Cells that make up the nutritional pathway at the chalazal end are small, closely packed, and rich in mitochondria. Its wall is thickened irregularly by heavy deposition of el ectron-translucent material and is interspersed with prominent bundles of plasmodesmata (Fig. 2). It seems likely that the inlet passage is predominantly symplastic in nature. 2. Wi thdrawal of cell contents from the nucellar tissue at the early stage is carried out by efflux of nuclear substance (karyoplasm) through enlarged openings on the nuclear envelop, and by exokaryosis of vesicles packed with ribosome-like granules (Fig. 6). These vesicles can then be trapped in the ER cavity. Breakdown of the endomembrane system follows next. A multitude of small vacuoles, vesicles (coated or not) impregnated with sap, fibrils and granules respectively, deformed mitocondria, chromatic aggregates, etc. can be found in suspension within the deteriorating cell (Fig. 7, 10). In addition, degradation of polysaccharides can also take place. Withdrawal of the cell content from shrinking nucellar layer and its flow into the antipodal section is through the ruptured wall where the cellulose skeleton is turned loose and fluffy at the opening. The protoplasmic fragments in transport are those structures of definite submicroscopic constitution, resulted from disassembly and disinteg ration of the protoplasm and from reassorment of protoplasmic constituents. 3. The antipo dal cells are separated from each other by partial walls riddled with cytoplasmic canals. The naked portion of the ceU can be in direct access to the invading fragments, which can be utilized and incorporated by the antipodal cell and participate in the building of new cell possibly by reorgnization (Fig. 12a) which may be a special mode of cell proliferation in antipodals. Amitosis of antipodal nucleus also has been observed (Text Fig. 3). Discussion is made in regards to the physiological significance of the nutritional supply in form of protoplasmic fragment and of the self-propelling mobility of the fragment. Alth ough the antipodals still proliferate to some extent even after fertilization, they meet the same fate as its predecessor, the nucellus, and soon vanish during the establishment of en dosperm. In food transport, the interconversion of polymer-monomer of saccarides, etc. is fre quently involved. In the present case of material transport, interconversion at higher levels plays a dominant role as shown in the assembly and disassembly of protoplasmic constituents and in the orgnization and disorgnization of ephemeral tissue.     

灵武长枣果实ATPase超微细胞化学定位和功能研究

采用ATPase超微细胞化学定位技术,研究灵武长枣果实不同发育阶段韧皮部和果肉库薄壁细胞ATPase分布特征,以明确灵武长枣果实ATPase超微细胞化学定位特征和功能。结果显示:(1)第一次快速生长期SE/CC复合体与周围的薄壁细胞有丰富的胞间连丝,形成共质体连续,韧皮部薄壁细胞之间有丰富的胞间连丝,ATPase反应物在韧皮部各细胞分布较少。(2)缓慢生长期ATPase反应物在韧皮部各细胞分布逐渐增加。(3)第二次快速生长期SE/CC复合体与周围的薄壁细胞缺乏胞间连丝,形成共质体隔离,韧皮薄壁细胞及果肉库薄壁细胞的胞间连丝较少,囊泡和膜泡在筛管、韧皮薄壁细胞和库薄壁细胞中很丰富,质膜、液泡膜、囊泡膜、细胞壁和胞间隙的ATPase活性较高。研究表明,果实在第一次快速生长期同化物从筛分子的卸出主要采取共质体途径,缓慢生长期同化物卸出时可能为共质体和质外体途径共存,第二次快速生长期则主要以质外体途径为主,证明果实不同发育阶段韧皮部同化物卸出路径存在差异。     

Transport of assimilates in the developing caryopsis of rice (Oryza sativa L.)

Assimilates entering the developing rice caryopsis traverse a short-distance pathway between the terminal sieve elements of the pericarp vascular bundle and the aleurone layer. The ultrastructure of this pathway has been studied. Sieve elements in the pericarp vascular bundle are smaller than their companion cells.The sieve elements show few connections with surrounding vascular parenchyma elements but are connected to companion cells by compound plasmodesmata. Companion cells, in turn, are connected to vascular parenchyma elements by numerous compound plasmodesmata present in wall thickenings. Assimilates leaving the sieve element — companion cell complex must laterally traverse cells of the pigment strand before they come into contact with the aleurone layer. The pigment strand cells have modified inner walls made up of a suberin-like material. This material may act as a permeability barrier isolating the apoplast from the symplast of the pigment strand. The walls of the pigment strand cells are traversed by numerous plasmodesmata. Water may be conducted to the endosperm through the isolated cell-wall system of the pigment strand while assimilates possibly move via plasmodesmata. High frequencies of plasmodesmata occur at the junction between the pigment strand and the nucellus and also between adjacent cells of the nucellus. By contrast, plasmodesmata are absent between the nucellus and the aleurone layer and also between the nucellus and the seed coat. A predominantly circumferential and symplastic transport pathway is likely between the pigment strand and nucellus. In view of the total absence of plasmodesmata between the nucellus and the aleurone layer assimilates entering the endosperm may have to cross the plasmalemma of the nucellus. It is possible that constraints to the flow of assimilates may occur in the short-distance pathway between the terminal sieve element — companion cell complexes and the endosperm, and this is discussed.     

Relation of beet yellows virus to the phloem and to movement in the sieve tube

In minor veins of leaves of Beta vulgaris L. (sugar beet) yellows virus particles were found both in parenchyma cells and in mature sieve elements. In parenchyma cells the particles were usually confined to the cytoplasm, that is, they were absent from the vacuoles. In the sieve elements, which at maturity have no vacuoles, the particles were scattered throughout the cell. In dense aggregations the particles tended to assume an orderly arrangement in both parenchyma cells and sieve elements. Most of the sieve elements containing virus particles had mitochondria, plastids, endoplasmic reticulum, and plasma membrane normal for mature sieve elements. Some sieve elements, however, showed evidence of degeneration. Virus particles were present also in the pores of the sieve plates, the plasmodesmata connecting the sieve elements with parenchyma cells, and the plasmodesmata between parenchyma cells. The distribution of the virus particles in the phloem of Beta is compatible with the concept that plant viruses move through the phloem in the sieve tubes and that this movement is a passive transport by mass flow. The observations also indicate that the beet yellows virus moves from cell to cell and in the sieve tube in the form of complete particles, and that this movement may occur through sieve-plate pores in the sieve tube and through plasmodesmata elsewhere.     

Electron Microscope Observations on Spore Formation in the True Slime Mold Didymium nigripes

SYNOPSIS. Developing and mature sporangia of the true slime mold Didymium nigripes were studied with the electron microscope to follow the course of spore formation. The sporangium forms from the plasmodium as a protoplasmic bleb which differentiates into a stalk and an apical sphere containing a mass of protoplasm. Nuclei within this protoplasmic mass undergo synchronous division (presumably meiosis). The division spindle forms within the nuclear membrane which is retained intact throughout the division; centrioles have not been observed at the spindle poles. At the same time the nuclei are dividing, the protoplasm cleaves to give ultimately uninucleate spheres—the incipient spores. Capillitial threads come to lie in the furrows created by the cleaving protoplasm. A wall consisting of an inner thick component and an outer thin component forms about each sphere. Cyto-chemical tests suggest that the inner wall of the spore is cellulose-containing and that the outer component might contain chitin.     

Distribution and frequency of plasmodesmata in relation to photoassimilate pathways and phloem loading in the barley leaf

Large, intermediate, and small bundles and contiguous tissues of the leaf blade of Hordeum tvulgare L. ‘Morex’ were examined with the transmission electron microscope to determine their cellular composition and the distribution and frequency of the plasmodesmata between the various cell combinations. Plasmodesmata are abundant at the mesophyll/parenchymatous bundle sheath, parenchymatous bundle sheath/mestome sheath, and mestome sheath/vascular parenchyma cell interfaces. Within the bundles, plasmodesmata are also abundant between vascular parenchyma cells, which occupy most of the interface between the sieve tube-companion cell complexes and the mestome sheath. Other vascular parenchyma cells commonly separate the thick-walled sieve tubes from the sieve tube-companion cell complexes. Plasmodesmatal frequencies between all remaining cell combinations of the vascular tissues are very low, even between the thin-walled sieve tubes and their associated companion cells. Both the sieve tube-companion cell complexes and the thick-walled sieve tubes, which lack companion cells, are virtually isolated symplastically from the rest of the leaf. Data on plamodesmatal frequency between protophloem sieve tubes and other cell types in intermediate and large bundles indicate that they (and their associated companion cells, when present) are also isolated symplastically from the rest of the leaf. Collectively, these data indicate that both phloem loading and unloading in the barley leaf involve apoplastic mechanisms.     

Plasmodesmatal distribution and frequency in vascular bundles and contiguous tissues of the leaf ofThemeda triandra

Small and intermediate vascular bundles and contiguous tissues of the leaf blade ofThemeda triandra var.imberbis (Retz.) A. Camus were examined with transmission and scanning electron microscopes to determine the distribution and frequency of plasmodesmata between various cell types. Plasmodesmata are most abundant at the mesophyll/bundle-sheath cell and bundle-sheath/vascular parenchyma cell interfaces, and their numbers decrease with increasing proximity to both thick- and thin-walled sieve tubes. Among cells of the vascular bundles, the greatest frequency of plasmodesmata occurs between vascular parenchyma cells, followed by that of plasmodesmata between vascular parenchyma cells and companion cells, and then by the pore-plasmodesmata connections between companion cells and thin-walled sieve tubes (sieve tube-companion cell complexes). The sieve tube-companion cell complexes of theT. triandra leaf are not isolated symplastically from the rest of the leaf and, in this respect, differ from their counterparts in theZea mays leaf. However, the thick-walled sieve tubes, like their counterparts inZea mays, lack companion cells and are symplastically connected with vascular parenchyma cells that about the xylem.Abbreviations SEM scanning electron microscope - TEM transmission electron microscope