Leaf vasculature in Zea mays L.

The vascular system of the Zea mays L. leaf consists of longitudinal strands interconnected by transverse bundles. In any given transverse section the longitudinal strands may be divided into three types of bundle according to size and structure: small, intermediate, large. Virtually all of the longitudinal strands intergrade structurally however, from one bundle type to another as they descend the leaf. For example, all of the strands having large-bundle anatomy appear distally as small bundles, which intergrade into intermediates and then large bundles as they descend the leaf. Only the large bundles and the intermediates that arise midway between them extend basipetally into the sheath and stem. Most of the remaining longitudinal strands of the blade do not enter the sheath but fuse with other strands above and in the region of the blade joint. Despite the marked decrease in number of longitudinal bundles at the base of the blade, both the total and mean cross-sectional areas of sieve tubes and tracheary elements increase as the bundles continuing into the sheath increase in size. Linear relationships exist between leaf width and total bundle number, and between cross-sectional area of vascular bundles and both total and mean cross-sectional areas of sieve tubes and tracheary elements.     

Leaf vasculature in sugarcane (Saccharum officinarum L.)

The vascular system of the leaves of Saccharum officinarum L. is composed in part of a system of longitudinal strands that in any given transverse section may be divided into three types of bundle according to size and structure: small, intermediate, and large. Virtually all of the longitudinal strands intergrade, however, from one type bundle to another. For example, virutually all of the strands having large bundle anatomy appear distally in the blade as small bundles, which intergrade into intermediates and then large bundles as they descend the leaf. These large bundles, together with the intermediates that arise midway between them, extend basipetally into the sheath and stem. Most of the remaining longitudinal strands of the blade do not enter the sheath but fuse with other strands above and in the region of the blade joint. Despite the marked decrease in number of bundles at the base of the blade, both the total and mean cross-sectional areas (measured with a digitizer from electron micrographs) of sieve tubes and tracheary elements increase as the bundles continuing into the sheath increase in size. Linear relationships exist between leaf width and total bundle number, and between cross-sectional area of vascular bundles and both total and mean cross-sectional areas of sieve tubes and tracheary elements.     

Structurally,phloem unloading in the maize leaf cannot be symplastic

Developing longitudinal vascular bundles of the leaf blades of maize (Zea mays L. cv. W273) were examined with the transmission electron microscope to determine the frequency of plasmodesmata between the sieve tubes and their neighboring cells. Of particular interest were the protophloem sieve tubes, the first sieve tubes to mature in importing (all large and some intermediate) bundles. The protophloem sieve tubes, most of which lack companion cells, intergrade structurally with the thin-walled metaphloem sieve tubes. Both the protophloem sieve tubes and the thin-walled metaphloem sieve tubes and their companion cells (the sieve tube-companion cell complexes) are virtually isolated symplastically from the rest of the leaf, precluding a symplastic mechanism of phloem unloading in the leaf blade of maize.     

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.     

STUDIES ON THE LEAF OF POPULUS DELTOIDES (SALICACEAE): QUANTITATIVE ASPECTS,AND SOLUTE CONCENTRATIONS OF THE SIEVE-TUBE MEMBERS

The vascular system of the leaf of Populus deltoides Bartr. ex Marsh, was examined quantitatively, and plasmolytic studies were carried out to determine the solute concentrations of sieve-tube members at various locations in the leaf. Both the total number and total crosssectional area of each cell type decreases with decreasing vein size. Although the proportion of phloem occupied by sieve tubes varies considerably from location to location, a linear relationship exists between cross-sectional area of the vascular bundles and both total and mean cross-sectional area of sieve tubes. Collectively, the cross-sectional area of all tertiary and minor veins feeding into a secondary exceeds the total cross-sectional area of sieve tubes at the base of that secondary. Moreover, the total volume of sieve tubes in the “catchment area” of a secondary vein is much greater than the total sieve tube volume of the secondary itself. Both tracheary elements and sieve-tube members undergo a reduction in both total and mean crosssectional area in the constricted zone at the base of the leaf. The plasmolytic studies revealed the presence of positive concentration gradients in sieve tubes of the lamina from the minor veins and tips of the secondaries to the bases of the secondaries and their associated subjacent midvein bundles and from the upper to lower portions of the median bundle of the midvein.     

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.     

Ultrastructure of and plasmodesmatal frequency in mature leaves of sugarcane

Vascular bundles and contiguous tissues of leaf blades of sugarcane (Saccharum interspecific hybrid L62–96) were examined with light and transmission electron microscopes to determine their cellular composition and the frequency of plasmodesmata between the various cell combinations. The large vascular bundles typically are surrounded by two bundle sheaths, an outer chlorenchymatous bundle sheath and an inner mestome sheath. In addition to a chlorenchymatous bundle sheath, a partial mestome sheath borders the phloem of the intermediate vascular bundles, and at least some mestome-sheath cells border the phloem of the small vascular bundles. Both the walls of the chlorenchymatous bundlesheath cells and of the mestome-sheath cells possess suberin lamellae. The phloem of all small and intermediate vascular bundles contains both thick- and thin-walled sieve tubes. Only the thin-walled sieve tubes have companion cells, with which they are united symplastically by pore-plasmodesmata connections. Plasmodesmata are abundant at the Kranz mesophyll-cell-bundlesheath-cell interface associated with all sized bundles. Plasmodesmata are also abundant at the bundle-sheathcell-vascular-parenchyma-cell, vascular-parenchyma-cellvascular-parenchyma-cell, and mestome-sheath-cell-vascular-parenchyma-cell interfaces in small and intermediate bundles. The thin-walled sieve tubes and companion cells of the large vascular bundles are symplastically isolated from all other cell types of the leaf. The same condition is essentially present in the sieve-tube-companion-cell complexes of the small and intermediate vascular bundles. Although few plasmodesmata connect either the thin-walled sieve tubes or their companion cells to the mestome sheath of small and intermediate bundles, plasmodesmata are somewhat more numerous between the companion cells and vascular-parenchyma cells. The thick-walled sieve tubes are united with vascular-parenchyma cells by pore-plasmodesmata connections. The vascular-parenchyma cells, in turn, have numerous plasmodesmatal connections with the bundle-sheath cells.This study was supported by National Science Foundation grants DCB 87-01116 and DCB 90-01759 to R.F.E. and a University of Wisconsin-Madison Dean's Fellowship to K. R.-B. We also thank Claudia Lipke and Kandis Elliot for photographic and artistic assistance, respectively.     

Leaf structure in relation to solute transport and phloem loading in Zea mays L.

Small and intermediate (longitudinal) vascular bundles of the Zea mays leaf are surrounded by chlorenchymatous bundle sheaths and consist of one or two vessels, variable numbers of vascular parenchyma cells, and two or more sieve tubes some of which are associated with companion cells. Sieve tubes not associated with companion cells have relatively thick walls and commonly are in direct contact with the vessels. The thick-walled sieve tubes have abundant cytoplasmic connections with contiguous vascular parenchyma cells; in contrast, connections between vascular parenchyma cells and thin-walled sieve tubes are rare. Connections are abundant, however, between the thin-walled sieve tubes and their companion cells; the latter have few connections with the vascular parenchyma cells. Plasmolytic studies on leaves of plants taken directly from lighted growth chambers gave osmotic potential values of about-18 bars for the companion cells and thin-walled sieve tubes (the companion cell-sieve tube complexes) and about-11 bars for the vascular parenchyma cells. Judging from the distribution of connections between various cell types of the vascular bundles and from the osmotic potential values of those cell types, it appears that sugar is actively accumulated from the apoplast by the companion cell-sieve tube complex, probably across the plasmalemma of the companion cell. The thick-walled sieve tubes, with their close spatial association with the vessels and possession of plasmalemma tubules, may play a role in retrieval of solutes entering the leaf apoplast in the transpiration stream. The transverse veins have chlorenchymatous bundle sheaths and commonly contain a single vessel and sieve tube. Parenchymatic elements may or may not be present. Like the thick-walled sieve tubes of the longitudinal bundles, the sieve tubes of the transverse veins have plasmalemma tubules, indicating that they too may play a role in retrieval of solutes entering the leaf apoplast in the transpiration stream.     

THE TIME COURSE OF SIEVE TUBE AND VESSEL REGENERATION AND THEIR RELATION TO PHLOEM ANASTOMOSES IN MATURE INTERNODES OF COLEUS

Quantitative counts of regenerative sieve tubes and vessels were made in a large number of samples of mature internode #5 of C. blumei, with concomitant study of the fine details of vascular regeneration and the occurrence of the normally developing phloem anastomoses. Such anastomoses were found in many of the plants, but their average number in the small regenerating area was low (viz., 0.9 ± 0.2). With the phloem anastomoses excluded from the counts, the time course of regeneration was clear cut—no strands completed their regeneration around the wound until three days after wounding. More regenerative sieve tubes completed their differentiation under all conditions than did regenerative vessels. The number of sieve tubes and vessels regenerated by four days was closely related to the number of preexisting bundles of that type of vascular cell that had been severed by the transverse wound. The ratio of bundles severed by the wound in the phloem to those in the xylem was 2.14, and the ratio of the regenerative sieve tubes to the regenerative vessels was 2.24. For both tracheary and sieve tube cells the initial regeneration was strongly polar (mostly above the wound), as expected from earlier IAA transport data. The path of tracheary regeneration was obviously related to that of the sieve tubes on the other side of the cambium.     

Vascular Anatomy of Angiospermous Leaves,with Special Consideration of the Maize Leaf

In this brief review an attempt has been made to discuss some of the important features of the vascular anatomy of angiospermous leaves, especially those related to assimilate transport. Accordingly, emphasis has been placed on the small or minor veins, which are closely related spatially to the mesophyll, and which play a major role in the uptake and subsequent transport of photosynthates from the leaf. The small veins are enclosed by bundle sheaths that intervene between the mesophyll and vascular tissues and greatly increase the area for contact with mesophyll cells. In the minor veins of dicotyledonous leaves, parenchymatic cells having organelle-rich protoplasts and numerous cytoplasmic connections with sieve elements dominate quantitatively. It is these so-called intermediary cells that apparently are directly involved with the loading of assimilates into the sieve elements. In the maize leaf the small and intermediate bundles have two types of sieve tubes, relatively thin-walled ones that have numerous cytoplasmic connections with companion cells, and thick-walled ones that lack companion cells but have numerous connections with vascular parenchyma cells. The companion cell-sieve tube complexes are virtually isolated symplastically from other cells of the vascular bundle and from the bundle sheath. Thick-walled sieve tubes similar to those in the maize leaf have been recorded in the leaves of other grasses.     

Loading and transport of assimilates in different maize leaf bundles

The loading and transport functions of vascular bundles in maize (Zea mays L.) leaf strips were investigated by microautoradiography after application of ¹⁴CO₂. The concentrations of ¹⁴C-contents in thin-walled sieve tubes of individual bundles in the loading and transport regions were determined by digital image analysis of silver-grain density over the sieve tubes and compared. In the loading region, relatively high concentrations of ¹⁴C-contents were found in the thin-walled sieve tubes of small bundles and in the small, thin-walled sieve tubes of the intermediate bundles; the concentration of ¹⁴C-label in large bundles was very low. In the transport region, at a transport distance of 2 cm, all of the small bundles contained ¹⁴C-assimilates, but generally less than the same bundles did in the loading region; by comparison, at that distance intermediate and large bundles contained two-to threefold more ¹⁴C-assimilates than the same bundles in the loading region. The lateral transfer of assimilates from smaller to larger bundles via transverse veins could be demonstrated directly in microautoradiographs. A reverse transport from larger to smaller bundles was not found. At a transport distance of 4 cm, all large and intermediate bundles were ¹⁴C-labeled, but many of the small bundles were not. Although all longitudinal bundles were able to transport ¹⁴C-asimilates longitudinally down the blade, it was the large bundles that were primarily involved with longitudinal transport and the small bundles that were primarily involved with loading.     

Leaf and inflorescence peduncle anatomy: a contribution to the taxonomy of Rapateaceae

The anatomy of leaves and inflorescence peduncles was studied in species of Monotrema (4), Stegolepis (1) and Saxofridericia (1), aiming to contribute to the taxonomy of Rapateaceae. The form and structure of leaf blade midrib and the form of the inflorescence peduncle are diagnostic characteristics for the studied species. Monotrema is distinguished by: epidermal and vascular bundle outer sheath cells containing phenolic compounds in both organs; leaf blade with palisade and spongy chlorenchyma, arm-parenchyma, and air canals between the vascular bundles; leaf sheath with phenolic idioblasts in the mesophyll; inflorescence peduncle with tabular epidermal cells and air canals in the cortex and pith. Such characteristics support the recognition of Monotremoideae, which includes Monotrema. Stegolepis guianensis is distinguished by thick-walled epidermal cells and a plicate chlorenchyma in both organs; leaf blade with subepidermal fiber strands in abaxial surface and a heterogeneous mesophyll; inflorescence peduncle with rounded epidermal cells, a hypodermis with slightly thick-walled cells, and a pith with isodiametric cells and vascular bundles. Saxofridericia aculeata is distinguished by papillate epidermal cells in both organs; unifacial leaf blade with subepidermal fiber strands in both surfaces and a regular chlorenchyma; leaf sheath with a hypodermis in both surfaces and fiber bundles in the mesophyll; inflorescence peduncle with an undefined cortex and a hypodermis with thick-walled cells. S. guianensis shares few characteristics with S. aculeata, supporting their placement in different tribes.     

Minor vein structure and sugar transport in Arabidopsis thaliana

Leaf and minor vein structure were studied in Arabidopsis thaliana (L.) Heynh. to gain insight into the mechanism(s) of phloem loading. Vein density (length of veins per unit leaf area) is extremely low. Almost all veins are intimately associated with the mesophyll and are probably involved in loading. In transverse sections of veins there are, on average, two companion cells for each sieve element. Phloem parenchyma cells appear to be specialized for delivery of photoassimilate from the bundle sheath to sieve element-companion cell complexes: they make numerous contacts with the bundle sheath and with companion cells and they have transfer cell wall ingrowths where they are in contact with sieve elements. Plasmodesmatal frequencies are high at interfaces involving phloem parenchyma cells. The plasmodesmata between phloem parenchyma cells and companion cells are structurally distinct in that there are several branches on the phloem parenchyma cell side of the wall and only one branch on the companion cell side. Most of the translocated sugar in A. thaliana is sucrose, but raffinose is also transported. Based on structural evidence, the most likely route of sucrose transport is from bundle sheath to phloem parenchyma cells through plasmodesmata, followed by efflux into the apoplasm across wall ingrowths and carrier-mediated uptake into the sieve element-companion cell complex. Received: 5 October 1999 / Accepted: 20 November 1999     

A Variation of C(4) Leaf Anatomy in Arundinella hirta (Gramineae)

The species Arundinella hirta L. posseses a striking variation of the leaf anatomy that is characteristic of C₄ grasses. In addition to a sheath of large, bright green cells around the vascular bundles, there are strands of large parenchyma cells which appear identical to the bundle sheath cells and which run parallel to the vascular bundles, but which are not associated with any vascular tissue. This species may be useful for studying the cellular compartmentalization associated with the C₄ pathway and should provide interesting material for determining the role of translocation in the functioning of the C₄ system.     

Composite bundles,the host/parasite interface in the holoparasitic angiosperms Langsdorffia and Balanophora (Balanophoraceae)

Composite bundles are not simply a type of vascular bundles, but an integrated host/parasite interface. We investigated their structure in tubers of Langsdorffia and Balanophora. Composite bundles in both genera have similar components: 1) a central mass of host vascular tissues among which are located large parasite transfer cells; 2) a sheath of parasite parenchyma surrounding the central host vascular tissues; 3) specialized conducting tissues in the sheath; and 4) apical meristems composed of both host and parasite meristematic cells. Sheath parenchyma is recognizable from parasite tuber matrix by having thinner cell walls, and, especially in Langsdorffia, by the presence of collapsed matrix cells between the bundle sheath and tuber matrix. Sheath-conducting tissues consist of densely cytoplasmic transfer cells and small sieve tube members; in Langsdorffia, tracheary elements are also present. These sheath bundles connect with vascular bundles of the tuber matrix. Direct host/parasite contact only occurs by means of parasite transfer cells in the composite bundles. There is no xylem-xylem contact at the host/parasite interface. Abundance of parasite transfer cells suggests that they play an important role in nutrient absorption and translocation.     

Anatomy of hydathodes and pigment disks in leaves of Ficus diversifolia (Moraceae)

In Ficus diversifolia , "mistletoe fig", hydathodes are scattered over the leaf lamina. They open to the adaxial surface via a circular, slightly depressed epidermal pad studded with 20–30 water pores. The epithem is a cylinder of small cells, irregular in shape, with conspicuous intercellular spaces, bounded by a close-fitting sheath of unpigmented cells. Hydathodes occur above characteristic junctions of three or more minor veins, from which short tracheary strands extend upward into the epithem and end blindly. Phloem does not seem to be involved. Unsuccessful attempts were made to induce guttation. Red pigment spots occur abaxially in the axils of major veins. They are compact internal disks of parenchyma cells containing granules of various sizes. A vascular bundle arches downward, passes over the disk, then curves upward to rejoin other bundles. The central large disk has a palisade epidermis of slender elongate cells separated from each other by cuticle; the other disks have a normal epidermis.     

Differentiation of bundle sheath,mesophyll, and distinctive cells in the C4 grass Arundinella hirta (Poaceae)

The C₄ grass Arundinella hirta is characterized by unusual leaf blade anatomy: veins are widely spaced and files of bundle-sheath-like cells, the distinctive cells, form longitudinal strands that are not associated with vascular tissue. While distinctive cells (DCs) appear to function like bundle sheath cells (BSCs), they differ developmentally in two ways: they are derived from ground meristem rather than procambium and they are formed 1–2 plastochrons later. This study describes ultrastructural features of differentiating of BSCs, DCs, and associated mesophyll cells (MCs) during leaf development. BSCs and DCs differ from adjacent MCs by undergoing earlier cell enlargement, greater rates of chloroplast enlargement, reduction of chloroplast thylakoids at late stages of differentiation, more extensive starch formation, greater wall thickening, and deposition of a suberin lamella. The precocious delimitation of the bundle sheath layer is reflected in earlier BSC enlargement and vacuole growth. Derivation of DCs from ground meristem is correlated with late developmental changes in chloroplast size, wall thickness, and plasmodesmatal density. Despite these differences in timing of events, particularly at early stages, the development of the specialized structural features of BSCs and DCs is essentially similar. Thus, proximity to vascular tissue appears to be nonessential for the coordination and regulation of BSC- and MC-specific developmental events.     

Taxonomic, evolutionary and ecological implications of the leaf anatomy of rhizomatous Iris species

The leaf anatomy of the rhizomatous Iris species with ensiform leaves and the related genera Pardanthopsis and Belamcanda is described. Their isobilateral leaves may or may not have a pseudo-dorsiventral structure. Variable characters of their leaf blades include: outline in transverse section, height and shape of papillae, form and structure of stomata, transverse section outline of marginal fibre strands and sclerenchy matous inner bundle sheath at phloem and xylem poles, forms of mesophyll arrangement, mesophyll structure and air canals, vascular bundle arrangement and the detailed structure of the larger vascular bundles, distribution of tannin, size and distribution of crystals. The taxonomic and ecological significance of these characters has been evaluated.
The anatomical characteristics of 25 supraspecific taxa in three genera are presented and compared in tables. The relationships and evolutionary position of these taxa are discussed. Each of the three subgroups within Iris appears to be correlated with a syndrome of anatomical characters. Some species currently of uncertain taxonomic position are discussed, and their classification based on anatomical data is suggested.
Some characters related to xeromorphy or helomorphy are mentioned.     

木立芦荟叶内维管束发育过程的研究

采用半薄切片和组织化学方法研究了木立芦荟（Aloe arhorescetzs）叶内维管束的发育过程,并着重于维管束鞘细胞和芦荟素细胞的来源及组织类型。结果表明：维管束由原形成层发育而来,但在分化原生韧皮部筛管时,其外侧仍保留一层原形成层细胞,以后分裂、增大成为特殊的大型薄壁细胞（芦荟素细胞）,芦荟索细胞啦属于韧皮部的一部分。而维管束鞘细胞则来源于基本分生组织,属于基本组织的范畴,与维管束不同源。     

芦荟叶内芦荟素细胞的发育和蒽醌类物质的积累

应用石蜡切片、半薄切片、组织化学和荧光显微镜观察相结合的方法研究了木立芦荟叶内芦荟素细胞的发生、发育以及其蒽醌类物质的积累过程。结果表明,在叶内原形成层束分化成维管束初期,原形成层束外侧的一层细胞发育成维管束鞘。原生韧皮部筛管产生时,其外方尚保留1—2层原形成层细胞,当后生韧皮部和木质部开始分化时,此层细胞分裂。在后生韧皮部和木质部发育成熟过程中,这些细胞体积逐渐增大,并液泡化,发育成为大型薄壁细胞(芦荟素细胞),位于筛管外侧。据此,芦荟叶维管束内的大型薄壁细胞的来源与韧皮部相同,属于特化的韧皮部薄壁组织细胞。用醋酸铅处理过的上述材料的切片观察表明,芦荟素细胞在细胞体积增大,并液泡化时,在液泡内出现蒽醌类物质沉淀物,在成熟细胞的大液泡中充满沉淀物,此时,在荧光显微镜下芦荟素细胞发出桔黄色荧光。可见,此种芦荟素细胞是芦荟叶内蒽醌类物质的主要储存场所。