Planta (1985)164:448-458

P l a n t a 9 Springer-Verlag 1985

Leaf vaseulature in Zea mays L. S.H.Russell and R.F.Evert* Departments of Botany and Plant Pathology, University of Wisconsin, Madison, WI 53706, USA

Abstract. 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. Key words: Leaf vasculature - Sieve tube (area) - Tracheary element (area) - Vascular bundle Zea (vasculature).

leaves. The first study (Colbert and Evert 1982) dealt with leaf vasculature in sugarcane. The maize leaf was selected for study because it is an important object of numerous studies on phloem loading and transport (e.g. Pitombeira et al. 1981 ; Thompson and Dale 1981 ; Eschrich and Burchardt 1982; Fritz et al. 1983) and photosynthesis (e.g. Chapman and Hatch 1981; Miranda et al. 1981 a, b; Stamp 1981a, b; Thiagrajah et al. 1981 ; Bethenod et al. 1982). Moreover, like sugarcane, maize is an NADP-malic enzyme C 4 plant and it is of interest to compare the vasculature of two not closely related species with similar photosynthetic pathways. The maize leaf has been the object of several anatomical investigations (Sharman 1942; Esau 1943; Kisselbach 1949; Miranda et al. 1981 a),~but in none of these were the functional relationships of the various-sized vascular bundles or the overall vasculature of the mature leaf considered. The principal objectives of our study were: (1) to reexamine the anatomy of the maize leaf; (2) to determine the relationships of the various-sized vascular bundles throughout the leaf; and (3) to determine whether a drop in conduction capacity, in terms of total sieve-tube cross-sectional area, takes place with a decrease in number of vascular bundles at the base of the blade.

Material and methods Plant material. The plants used in this study were grown from grain of Zea mays L. hybrid W273 (Olds Seed, Madison, Wis.,

Introduction

This paper on maize represents the second detailed study undertaken to provide qualitative and quantitative information on the vasculature of grass * To whom correspondence should be addressed

USA). Most were grown in a Conviron E8VH growth chamber (Controlled Environments, Winnipeg, Canada) with a 16/8 h light/dark period (1000 gmol m-2 s-1), 65/80% relative humidity, and at 25/15~ C, in Pro-Mix A (Premier Brands, New York, N.Y., USA). The plants were watered approximately four times a week with a modified, 0.5-strength Hoagland's solution (Johnson et al. 1957). Some plants were grown in the

S.H. Russell and R.F. Evert: Leaf vasculature in Zea greenhouses of the Department of Botany, University of Wisconsin, Madison.

Light microscopy. The pattern of longitudinal bundles and the length and density of transverse bundles were determined from clearings of several whole leaves, ranging in length from 13 to 36 cm. Some leaf clearings were made according to the method of Shobe and Lersten (1967), except 50% chlorine bleach was used instead of chloral hydrate, while others were made according to the method of Boke (1970). Leaf tissue to be serially sectioned was cut into 5-mm-long samples and fixed in either ethanol:glacial acetic acid:formaldehyde (FAA) for 24 h, or chromic acid : acetic acid : formaldehyde (Craf III) for 3~4 d (Sass 1958). Dehydration in a graded tertiary-butanol series or ethanol-xylene series was followed by infiltration and embedment in Paraplast (Lancer, St. Louis, Mo., USA). Sections were cut at 7-20 gm on a rotary microtome. Staining was with toluidine blue 0 (0.5% in H 2 0 ) alone or in combination with brilliant green (0.5% in H 2 0 ) or basic fuchsin (Humphrey and Pittman 1974). (All stains: Allied Chemical, New York, N.Y., USA).

Electron microscopy. Leaf tissue for electron microscopy was fixed in 6% glutaraldehyde in 0.05 M cacodylate buffer, pH 7.2, for 6 h at 4 ~ C, and post-fixed in 2% osmium tetroxide in 0.05 M cacodylate buffer overnight in a refrigerator. The tissue was dehydrated in a graded ethanol series and infiltrated and embedded in Spurr's epoxy resin (Spurr 1969). Thin sections were cut with a diamond knife on a Porter-Blum MT-2 ultramicrotome (Sorval, Newton, Conn., USA), stained with uranyl acetate and lead citrate, and viewed and photographed with a Hitachi (Tokyo, Japan) HU-11C microscope.

Quantitative data. A quantitative description of the vasculature of the maize leaf was produced from free-hand cross-sections of fresh tissue and from cross-sections of Paraplast-embedded material. Seventy-four blade and 61 sheath cross-sections from 12 fully expanded leaves were used. The leaves ranged from 22 to 77 cm in total length, and included the fourth, fifth, and sixth visible leaves from the top of the plant and the second through fifth leaves from the coleoptile. Data were collected and expressed as in the Colbert and Evert (1982) study of the sugarcane leaf, with the location of a cross-section being expressed as percent of total leaf length. Zero percent was defined as the base of the sheath, 100% as the tip of the blade.

449 because of the variability in the percentage of leaf length contributed by the sheath (see Results and discussion Morphology). In calculating the total sieve-tube and tracheary-element areas, these intervals were handled in the same manner as was the 20-30% length interval in the sugarcane study (Colbert and Evert 1982).

Results and discussion

Morphology The maize leaf consists of two parts: the sheath, which envelops the stem with overlapping margins, and the blade. The sheath comprises 25-50% of leaf length, varying with the leaf's total length and its position on the stem; it comprises up to 50% of the length of smaller, earlier-formed leaves, and 20-30% of the larger, later-formed leaves. The sheath is widest at its base and narrowest where it merges with the blade. The linear lanceolate blade is widest at about 55% leaf length (from the base of the leaf), and tapers toward both its base and tip, though more sharply towards the acuminate tip. A midrib, projecting from the lower surface of the blade, is discernible fairly close to the leaf tip and becomes increasingly prominent towards the sheath. The midrib may extend basipetally into the sheath but becomes less prominent as it descends the sheath and finally disappears. For much of their length, the vascular bundles of the sheath are associated with ribs on their abaxial surfaces (Figs. 3, 4). The junction of the blade and sheath is termed the blade joint or dewlap, and is characterized by the presence of a ligule and auricles, although the latter are weakly developed. The junction of the sheath and stem is termed the sheath joint.

Percentage air space. The percentage of total leaf volume represented by air space was measured in three leaves using the method of Johnston (1977) as modified by Colbert and Evert (1982).

Cross-sectional area data. The procedures used in measurement of the cross-sectional areas of vascular bundles and of their sieve tubes and tracheary elements were similar to those utilized by Colbert and Evert (1982), with the following exceptions: (1) Measurements were made from electron micrographs (with a digitizer) of 10 transverse bundles; 26 small, 12 intermediate, and 10 large blade bundles; and 12 intermediate and 10 large sheath bundles. (2) The blade tissue was taken from the portion of each leaf estimated from the quantitative data to have the greatest number of bundles ( m e a n = 6 5 % total leaf length), rather than from the widest part of each leaf blade. (3) The total number of each longitudinal bundle type was counted in a leaf cross-section taken immediately below the regions sampled for electron microscopy and area measurements in both blade and sheath. (4) Both blade and sheath cross-sections occurred in the 20-30, 30-40, 40-50 and 50-60% length intervals

General anatomy of the blade The anatomy of the maize leaf is typical of that of C~ grasses (Esau 1977), with the mesophyll radially arranged around the chlorenchymatous bundle sheath (Fig. 1). Moreover, as is typical of an NADP-malic enzyme species, the chloroplasts are centrifugally arranged within the bundle-sheath cells (Hatch et al. 1975). In most of the blade, only two layers of mesophyll cells intervene between adjacent longitudinal bundles; hence, the maximum lateral cell count (Hattersley and Watson 1975) is two. In the midrib, the ground tissue is composed mainly of large, so-called colorless cells (Fig. 2), which account for most of the thickness of the blade in this region, and contain relatively few,

450

S.H. Russell and R.F. Evert : Leaf vasculature in Zea

Figs. 1, 2. Transverse sections through blade of maize leaf. x 135 ; bar = 75 gm. Arrows point to hypodermal sclerenchyma associated with large and intermediate bundles. Fig. 1. From flat portion of blade. The intermediate bundle (far left) is separated from the large bundle (far right) by seven small bundles. Fig. 2. Midrib from lower half of blade. The large bundle in the median position is the median bundle. In the midrib, large colorless cells (CC) intervene between the vascular bundles and upper epidermis (not shown)

small chloroplasts. The mean interveinal distance (from middle of bundle to middle of bundle) is 0.120 m m (Table 1). The mesophyll cells of the blade are deeply lobed and have numerous intercellular spaces among them. Together, the substomatal chambers and network of air (intercellular) spaces result in the blade being about 14% air space, as determined by the method of Johnston (1977), a value greater than the 10.25% air space calculated by Byott (1976) for the leaf blade of maize. As seen in any given transverse section, three types, or orders, of longitudinal vascular bundle may be recognized in the blade of the maize leaf (Esau 1943; Ellis 1976; Evert et al. 1977): large, intermediate, and small. The longitudinal bundles are interconnected by numerous, small transverse bundles.

As seen in transverse section, the pattern of longitudinal bundles in the blade is somewhat variable. Commonly, one to three intermediate bundles occur between large bundles. Intermediate bundles, in turn, are separated from one another and from large bundles by two to seven small bundles (for example Fig. 1). In contrast, intermediate bundles may outnumber small bundles in the midrib. There, large bundles (including the median bundle) commonly are separated by one to three bundles (for example Fig. 2). In the flat portion of the blade, both intermediate and large blade-bundles are associated with strands or girders of hypodermal sclerenchyma, the large bundles on both ad- and abaxial surfaces, and the intermediate bundles on one or both surfaces (Figs. 1, 2). In the midrib, adaxial sclerenchyma strands occur opposite only the large bun-

S.H. Russell and R.F. Evert: Leaf vasculature in Zea Table 1. Comparison of various parameters in blade and sheath of the maize leaf Blade % total leaf length % air space Interveinal distance (ram) Longitudinal bundles (cm cm- z leaf area) Number of transverse bundles cm- 2 Mean length of transverse bundles (mm) Total bundle length (cm cm- z leaf area) % length longitudinal bundles % length transverse bundles a

Sheath

66.5 _+9.3a 33.5 __.9.3 13.7 _+6.0 10.1 +4.1 0.120 _+0.027 0.455_+0.129 83.3 22.0 624

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0.096 + 0.029

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89.3

24.8

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dles, and are separated from them by several layers of colorless cells. In general, sclerenchyma is most strongly developed in the midrib region of the blade (Fig. 2). A small sclerenchyma strand occurs at the leaf margin at all levels of the blade. The anatomy of the blade intergrades with that of the sheath in the region of the blade joint. At the blade joint, the ground tissue is composed largely of colorless cells and the bundle-sheath cells contain relatively few chloroplasts; hence, the Kranz anatomy typical of the blade is lacking in the blade joint. In addition, in the blade joint, sclerenchyma is largely replaced by collenchyma and most of the strands with intermediate-bundle anatomy intergrade into small bundles which then fuse with other, adjacent strands near the blade joint (Fig. 5b). General anatomy of the sheath The leaf sheath, like the blade joint, does not exhibit a Kranz anatomy (Figs. 3, 4). The vascular bundles of the sheath are bordered by bundle sheaths consisting of relatively thin-walled, sclerifled cells with variable numbers of chloroplasts distributed along all walls. The bundle sheaths are interrupted along their abaxial surfaces by the hypodermal sclerenchyma strands. Near blade and sheath joints the chloroplasts of the bundle-sheath cells are small and inconspicuous. Between these two regions, however, the bundle-sheath cells contain relatively large chloroplasts. Chloroplasts are

451 found in all bundle-sheath cells associated with the intermediate bundles, but occur largely opposite the phloem of the large bundles. The ground-tissue cells of the leaf sheath also contain variable numbers of chloroplasts. Virtually all ground-tissue cells in narrow portions of the leaf sheath and those bordering chlorophyllous bundle-sheath cells throughout the leaf sheath contain numerous chloroplasts (Figs. 3, 4) which are especially conspicuous in fresh material. Elsewhere - largely in thicker portions of the leaf sheath the ground tissue consists mostly of colorless cells (Fig. 4), with relatively few, small chloroplasts. It is pertinent to note that the chloroplasts of the bundle-sheath cells and contiguous ground-tissue cells are similar in appearance, in that all have well-developed grana. In the blade, the bundlesheath chloroplasts have poorly-developed grana (Evert et al. 1977). The vascular bundles in the sheath are separated by seven to twelve ground-tissue cells, and the mean interveinal distance is 0.455 mm (Table 1). The ground-tissue cells of the sheath have numerous schizogenous intercellular spaces among them. The sheath consists of about 10% air space. Three types of longitudinal bundle may also be recognized in the leaf sheath of maize. Bundles corresponding to the large and intermediate types of the blade are common throughout the sheath, while small bundles are found only in upper (near the blade joint) and lower (near the sheath joint) portions of the sheath. In the upper portion of the sheath the small bundles occur largely near the margins. Transverse bundles are also present in the sheath. As seen in transverse section, the pattern of longitudinal bundles in the sheath is more regular than that of the blade. Generally, large bundles alternate with intermediate ones (Figs. 3, 4). In the upper portion of the sheath, the small bundles are found interspersed among the large and intermediate bundles largely near the leaf margins. Most bundles in the sheath are associated with large amounts of sclerenchyma, particularly on their abaxial surfaces (Figs. 3, 4). Adaxially, a vascular bundle may or may not be contiguous to a sclerenchyma strand, as several layers of colorless cells may intervene between the two. As in the blade joint, the hypodermal sclerenchyma is largely replaced by collenchyma in the sheath joint. Vascular bundles Strictly speaking, a vascular bundle is a strand-like part of the vascular system, consisting solely of

452

S.H. Russell and R.F. Evert: Leaf vasculature in Zea

Figs. 3, 4. Transverse sections through ribbed portion of sheath, x 135; bar=75 gin. Fig. 3. Portion of sheath about halfway between leaf margin and median bundle. Arrows point to hypodermal sclerenchyma. Fig. 4. Mid-portion of sheath showing median bundle flanked on either side by intermediate bundles. CC, Colorless cells; arrowheads point to bundle-sheath cells

xylem and phloem (Metcalfe 1960; Esau 1977). In the following descriptions, however, bundle sheaths and hypodermal sclerenchyma will be considered together with the vascular bundles.

Large bundles. Large bundles are characterized by the presence of large metaxylem vessels on either side of the protoxylem or protoxylem lacuna (Figs. 1-4). Protophloem is also present, but in

mature bundles it is crushed (Esau 1943); hence, the only conducting phloem in such bundles is metaphloem. The metaphloem in the large blade bundles contains only thin-walled sieve tubes, except for portions of strands intergrading between intermediate and large bundle anatomy (see Vaseulature of the leaf). Most large blade-bundles are bordered by a chlorenchymatous bundle sheath, which is often

S.H. Russell and R.F. Evert: Leaf vasculaturein Zea interrupted by hypodermal sclerenchyma on both adaxial and abaxial surfaces. In the midrib of the blade and thoughout the sheath, the bundle-sheath cells are sclerified, and only those bordering mesophyll cells in the blade or chlorophyllous cells in the sheath have conspicuous chloroplasts. Large bundles of the midrib and of most of the sheath are directly associated with well-developed hypodermal sclerenchyma on only their abaxial surfaces (Figs. 2-4), being separated from generally poorlydeveloped hypodermal sclerenchyma strands on their adaxial surfaces by several layers of colorless cells. Near the base of the sheath, hypodermal sclerenchyma is lacking entirely along the adaxial surface, Intermediate bundles. Unlike large bundles, intermediate bundles lack large metaxylem vessels and protoxylem but may have protophloem. Whereas the metaphloem of intermediate blade-bundles contains both thick- and thin-walled sieve tubes that of intermediate sheath-bundles may contain only thin-walled sieve tubes. Most intermediate blade-bundles are completely surrounded by a chlorenchymatous bundle sheath and are associated with hypodermal sclerenchyma either adaxialty or abaxially, or both. The bundle-sheath cells of intermediate sheath-bundles tend to resemble the bundle-sheath cells bordering the phloem of large sheath-bundles in corresponding portions of the leaf sheath. In addition, the bundle sheaths of intermediate sheath-bundles are almost always interrupted abaxially by hypodermal sclerenchyma (Figs. 3, 4). The chlorenchymatous bundle-sheath cells of intermediate bundles are always bordered by mesophyll or chlorphyllous cells. Small bundles. Small bundles consist entirely of metaxylem and metaphloem and always have both thick- and thin-walled sieve tubes (Evert et al. 1978). Small blade-bundles are completely surrounded by chlorenchymatous bundle sheaths which in turn are completely encircled by mesophyll cells (Figs. 1, 2). As mentioned previously, few small bundles occur in the leaf sheath. Small bundles in the sheath rarely are associated with chlorenchymatous bundie sheaths. Instead, they generally are enclosed by a layer of compactly arranged achlorophyllous cells. Transverse bundles. As mentioned previously, longitudinal bundles are interconnected by transverse bundles in both blade and sheath. The transverse

453 bundles usually have a single file of vessel members on the upper side of the bundle and a single file of sieve-tube members below the vessel members and in direct contact with them (Evert et al. 1978). Parenchymatous elements may or many not be associated with the sieve-tube members. Some bundles lack sieve-tube members. The transverse bundles are surrounded by a single layer of chlorenchymatous bundle-sheath cells (Evert et al. 1978). Transverse bundles are less numerous in the leaf sheath than in the blade (Table 1). Nevertheless, the transverse bundles comprise a greater percentage of the total bundle length in the sheath than in the blade, 11.5% and 6.7%, respectively (Table 1). Total length of transverse bundles per cm 2 in the blade is almost twice that in the sheath but, because of the greater density (cm cm-2) of longitudinal bundles in the blade, the transverse bundles there represent a smaller percentage of total bundle length.

Vasculature of the leaf

Both the total number of longitudinal vascular bundles (Fig. 6) and the percent of the total represented by each bundle type (Fig. 7) vary greatly over the length of the leaf. The total number of longitudinal bundles is, however, closely correlated with leaf width in both blade (r = 0.937) and sheath (r=0.842) (Fig. 8). Combining this information with that obtained from leaf clearings and serial transverse sections, it was possible to determine the interrelationships among vascular bundles along their longitudinal course from the tip to the base of the leaf. It is pertinent to note that the following account of the vasculature is purely descriptive. No developmental aspects are implied. The first bundle encountered at the tip of the blade is the median bundle, which is continuous through the entire length of the leaf (Fig. 5 a). At the leaf tip (100% leaf tength), this strand has the appearance of a small bundle, but it assumes the anatomy of an intermediate and then a large bundle within several millimeters of the leaf tip. Just short of the leaf tip two new strands diverge from either side of the median bundle. These new strands, which have small-bundle anatomy, bifurcate, and each of the inner branches assumes the anatomy of an intermediate and then a large bundle as it descends the blade (Fig. 5 a). The outer branches continue downward as small bundles near the leaf margin, bifurcating again and again to form new branches, the inner ones first assuming the structure of intermediate and then large

S.H. Russell and R.F. Evert: Leaf vasculature in Zea

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a Fig. S a, b. Diagrams of leaf vasculature in the blade and sheath of maize. Transverse bundles excluded, a Interrelationships of large bundles (heavy lines). Distally these longitudinal strands first appear as small bundles (light lines). With the exception of the median bundle, these small bundles bifurcate, the inner branches intergrading into intermediate (broken lines) and then

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Fig. 8. Relation between total number of bundles and width of blade ( x ) and sheath (~x)in the maize leaf. Regression equation for blade is y=6.02 x+30.87, and that for sheath, y = 1.14 x+23.48 large bundles as they descend the blade, h Pattern of longitudinal strands between two large bundles. (Arrows indicate region of blade joint.)

S.H. Russell and R.F. Evert: Leaf vasculature in Zea bundles as they continue basipetally to the base of the leaf. It is in this manner that all large bundles belonging to this system of strands are present by about 60% leaf length. This system of strands is basically similar to that comprising the large bladebundles of the sugarcane leaf (Colbert and Evert 1982). The majority of longitudinal strands in the blade do not begin as a bifurcation as described above. Instead, at their distal ends most at first resemble transverse bundles (often more or less Yshaped, with the upper branches of the Y each associated with a different longitudinal strand) that turn downward and follow a course parallel to the longitudinal strands rather than connecting adjacent longitudinal strands. Each of these strands soon assumes the anatomy of a small bundle as it descends the blade, and most of them end in the reverse manner; i.e. they assume the anatomy of transverse bundles, turn toward an adjacent longitudinal strand, and fuse with it (Fig. 5b). Considerable variability exists in the lengths of such small bundles. The first strand to appear distally between successive large bundles intergrades from transverseto small- and then intermediate-bundle anatomy, and continues downward to the sheath joint as an intermediate bundle. A few of these strands those nearest the median bundle - may assume large-bundle anatomy in the lower portion of the sheath (Fig. 5b). The next two strands to appear distally between successive large bundles occur on either side of the first strand (Fig. 5b). These strands also intergrade into intermediate bundles but, rather than continuing basipetally into the sheath as intermediates, assume the anatomy of small and then transverse bundles and fuse with adjacent longitudinal strands near the blade joint. As can be seen in Fig. 5b, most of the strands between successive large bundles maintain smallbundle anatomy throughout their length (except at their distal and proximal ends, where they resemble transverse bundles). Of such strands, those initiated nearer the leaf tip tend to extend farther down the blade than those initiated farther from the leaf tip. Almost all of these strands fuse with adjacent longitudinal strands before reaching the sheath; few enter the sheath. Moreover, the strands with largely small-bundle anatomy located near the midrib fuse with other longitudinal strands by 70% total leaf length, while those located nearer the margin tend to fuse with other longitudinal strands near or at the blade joint. This tends to impart a slight chevron pattern to the system of large blade-bundles (Fig. 5 a).

455 Most blade bundles are initiated above 65% leaf length, and fusion of bundles is most frequent near the midrib and near and in the region of the blade joint. Initiation and fusion of bundles may occur, however, in any portion of the leaf. This is quite a different picture of maize leaf vasculature than that depicted by Kisselbach (1949; his Fig. 19.2), in which virtually all fusion of blade bundles occurs just above the blade joint. Thus, in the maize leaf, as in the sugarcane leaf (Colbert and Evert 1982), virtually all longitudinal strands intergrade structurally from one bundle type to another. Moreover, in the leaves of both grasses it is the system of large bundles and the intermediate bundles that arise midway between successive large bundles that extend basipetally into the sheath and on into the stem as leaf traces. A few new small bundles may arise near the base of the sheath, resulting in a slight increase in the total number of bundles there (Fig. 6). Results of a recent microautoradiographic study of phloem loading and transport in the maize leaf (Fritz et al. 1983) indicate that, while the small and intermediate blade-bundles play the major role in the collection of photosynthates and in phloem loading, the large bundles are primarily involved with the transport of photosynthates out of the leaf. Apparently photosynthates are transported from the smaller bundles to the larger ones via the numerous transverse veins interconnecting them. Knowledge of the vasculature of the leaf supports those results. Inasmuch as the pathway followed by photosynthates out of a leaf is determined in large part by the canalizing effect of the vascular organization of the leaf, the vasculature of both maize and sugarcane leaves necessitates the transfer of photosynthates from the smaller, largely source bundles to the bundles that extend basipetally into the sheath and stem, namely the large blade-bundles and the intermediates that arise midway between them.

Cross-sectional area o f vascular tissues

The mean cross-sectional areas of vascular bundles and of their sieve tubes and tracheary elements are given in Table 2. In the leaves used to measure cross-sectional area, the average total sieve-tube area was estimated to be about 27 000 gm 2 in the portion of the blade with the greatest number of bundles (65% total leaf length), and 79000 gm 2 in the portion of the sheath with the fewest number of bundles (15% total leaf length). Tracheary-element area was estimated to the about 44000 gm 2

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cross-sectional areas were calculated at ten locations, from 95% to 5% total leaf length. As can be seen in Fig. 9, the total cross-sectional area of both sieve tubes and tracheary elements increases from the tip of the blade to the base of the sheath. Even though the number of vascular bundles drops dramatically (Fig. 6), the total cross-sectional area of the conducting tissues increases. The sieve-tube areas plotted in Fig. 9 include both thick- and thin-walled sieve-tube areas. A plot including only thin-walled sieve-tube area is very similar to that illustrated in Fig. 9 because the area of thick-walled sieve tubes represents only a relatively small percentage of the total sieve-tube area in each bundle type (Table 2). A close correlation was found between the cross-sectional area of vascular bundles and both total (r=0.991) and mean cross-sectional area of thin-walled sieve tubes (r = 0.938) (Fig. 10). A similar correlation was found between bundle area and both total cross-sectional area (r = 0.993) and mean cross-sectional area (r=0.970) of tracheary elements. (The regression equation for total area of tracheary elements is y = 0.396 x - 5 3 . 1 8 , and that for mean area of tracheary elements, y---0.44 x + 44.43). Close correlations were also found between the same parameters in sugarcane (Colbert and Evert 1982). Because the transverse bundles are very short compared with the longitudinal bundles, any comparison of the cross-sectional areas of their con-

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and 110000 gm2 in the blade and sheath, respectively. In order to achieve some estimate of the total sieve-tube and tracheary-element areas at different locations within the leaf, the mean cross-sectional areas estimated for each bundle type from the portion of the blade with the greatest number of bundles ( m e a n = 6 5 % total leaf length) and from the narrowest portion of the leaf (in the sheath below the blade joint; mean = 35% total leaf length) were combined with the data from the 12 leaves used to determine the quantitative aspects of leaf vasculature. Total sieve-tube and tracheary-element

Table 2. Mean cross-sectional areas (ttm 2) of vascular bundles and of their sieve tubes (STs) and tracheary elements in leaf of maize Parameter

Blade Transverse

Number of bundles measured

10

Mean total bundle area Mean total cell area per bundle Thin-walled STs Thick-walled STs Tracheary elements Mean individual cell area per bundle Thin-walled STs Thick-walled STs Tracheary elements Mean number of cells per bundle Thin-walled STs Thick-walled STs Tracheary elements Mean total ST area per bundle % thin-walled STs % thick-walled STs

44.4 199.0 44.4 ] 99.0 1.0 1.0 44.4 100

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26

12

10

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10

494

225

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2431

23036

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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...
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