The Role of the Epidermis in the Control of Elongation Growth in Stems and Coleoptiles

The peripheral cell wall(s) of stems and coleoptiles are 6 to 20 times thicker than the walls of the inner tissues. In coleoptiles, the outer wall of the outer epidermis shows a multilayered, helicoidal cellulose architecture, whereas the walls of the parenchyma and the outer wall of the inner epidermis are unilayered. In hypocotyls and epicotyls both the epidermal and some subepidermal walls are multilayered, helicoidal structures. The walls of the internal tissues (inner cortex, pith) are unilayered, with cellulose microfibrils oriented primarily transversely. Peeled inner tissues rapidly extend in water, whereas the outer cell layer(s) contract on isolation. This indicates that the peripheral walls limit elongation of the intact organ. Experiments with the pressure microprobe indicate that the entire organ can be viewed as a giant, turgid cell: the extensible inner tissues exert a pressure (turgor) on the peripheral wall(s), which bear the longitudinal wall stress of the epidermal and internal cells. Numerous studies have shown that auxin induces elongation of isolated, intact sections by loosening of the growth-limiting peripheral cell wall(s). Likewise, the effect of light on reduction of stem elongation and cell wall extensibility in etiolated seedlings is restricted to the peripheral cell layers of the organ. The extensible inner tissues provide the driving force (turgor pressure), whereas the rigid peripheral wall(s) limit, and hence control, the rate of organ elongation.     

The epidermal-growth-control theory of stem elongation: an old and a new perspective

The botanist G. Kraus postulated in 1867 that the peripheral cell layers determine the rate of organ elongation based on the observation that the separated outer and inner tissues of growing stems spontaneously change their lengths upon isolation from each other. Here, we summarize the modern version of this classical concept, the "epidermal-growth-control" or "tensile skin" theory of stem elongation. First, we present newly acquired data from sunflower hypocotyls, which demonstrate that the expansion of the isolated inner tissues is not an experimental artefact, as recently claimed, but rather the result of metabolism-independent cell elongation caused by the removal of the growth-controlling peripheral walls. Second, we present data showing that auxin-induced elongation of excised stem segments is attributable to the loosening of the thick epidermal walls, which provides additional evidence for the "epidermal-growth-control concept". Third, we show that the cuticle of aerial organs can be thin and mechanically weak in seedlings raised at high humidity, but thick and mechanically important for organs growing under relatively dry air conditions. Finally, we present a modified model of the "tensile skin-theory" that draws attention to the mechanical and physiological roles of (a) the thickened, helicoidal outer cell walls, (b) the mechanical constraint of a cuticle, and (c) the interactions among outer and inner cell layers as growth is coordinated by hormonal signals.     

Cell elongation in Arabidopsis hypocotyls involves dynamic changes in cell wall thickness

Field-emission scanning electron microscopy was used to measure wall thicknesses of different cell types in freeze-fractured hypocotyls of Arabidopsis thaliana. Measurements of uronic acid content, wall mass, and wall volume suggest that cell wall biosynthesis in this organ does not always keep pace with, and is not always tightly coupled to, elongation. In light-grown hypocotyls, walls thicken, maintain a constant thickness, or become thinner during elongation, depending upon the cell type and the stage of growth. In light-grown hypocotyls, exogenous gibberellic acid represses the extent of thickening and promotes cell elongation by both wall thinning and increased anisotropy during the early stages of hypocotyl elongation, and by increased wall deposition in the latter stages. Dark-grown hypocotyls, in the 48 h period between cold imbibition and seedling emergence, deposit very thick walls that subsequently thin in a narrow developmental window as the hypocotyl rapidly elongates. The rate of wall deposition is then maintained and keeps pace with cell elongation. The outer epidermal wall is always the thickest ( approximately 1 mum) whereas the thinnest walls, about 50 nm, are found in inner cell layers. It is concluded that control of wall thickness in different cell types is tightly regulated during hypocotyl development, and that wall deposition and cell elongation are not invariably coupled.     

Tissue stresses in growing plant organs

Rapidly growing plant organs (e.g. coleopties, hypocotyls, or internodes) are composed of tissues that differ with respect to the thickness, structure, and extensibility of their cell walls. The thick, relatively inextensible outer wall of the epidermal cells contains both transverse and longitudinally oriented cellulose-microfibrils. The orientation of microfibrils of the thin, extensible walls of the parenchyma cells seems to be predominantly transverse. In many growing organs (i.e. leafstalks), the outer epidermal wall is supported by a thickened inner epidermal wall and by thick-walled subepidermal collenchyma tissue. Owing to the turgor pressure of the cells the peripheral walls are under tension, while the extensible inner tissue is under compression. As a corollary, the longitudinal tensile stress of the rigid peripheral wall is high whereas that of the internal walls is lowered. The physical stress between the tissues has been described by Sachs in 1865 as 'tissue tension'. The term 'tissue stress'. however, seems to be more appropriate since it comprises both tension and compression. Hitherto no method has been developed to measure tissue stresses directly as force per unit cross-sectional area. One can demonstrate the existence of tissue stresses by separation of the tissues (splitting, peeling) and determining the resulting strain of the isolated organ fragments. Based on such experiments it has been shown that rapid growth is always accompanied by the existence of longitudinal tissue stresses.     

Interaction between wall deposition and cell elongation in dark-grown hypocotyl cells in Arabidopsis

A central problem in plant biology is how cell expansion is coordinated with wall synthesis. We have studied growth and wall deposition in epidermal cells of dark-grown Arabidopsis hypocotyls. Cells elongated in a biphasic pattern, slowly first and rapidly thereafter. The growth acceleration was initiated at the hypocotyl base and propagated acropetally. Using transmission and scanning electron microscopy, we analyzed walls in slowly and rapidly growing cells in 4-d-old dark-grown seedlings. We observed thick walls in slowly growing cells and thin walls in rapidly growing cells, which indicates that the rate of cell wall synthesis was not coupled to the cell elongation rate. The thick walls showed a polylamellated architecture, whereas polysaccharides in thin walls were axially oriented. Interestingly, innermost cellulose microfibrils were transversely oriented in both slowly and rapidly growing cells. This suggested that transversely deposited microfibrils reoriented in deeper layers of the expanding wall. No growth acceleration, only slow growth, was observed in the cellulose synthase mutant cesA6(prc1-1) or in seedlings, which had been treated with the cellulose synthesis inhibitor isoxaben. In these seedlings, innermost microfibrils were transversely oriented and not randomized as has been reported for other cellulose-deficient mutants or following treatment with dichlorobenzonitrile. Interestingly, isoxaben treatment after the initiation of the growth acceleration in the hypocotyl did not affect subsequent cell elongation. Together, these results show that rapid cell elongation, which involves extensive remodeling of the cell wall polymer network, depends on normal cellulose deposition during the slow growth phase.     

THE STRUCTURE OF THE PRIMARY EPIDERMAL CELL WALL OF AVENA COLEOPTILES

The primary walls of epidermal cells in Avena coleoptiles ranging in length from 2 to 40 mm. have been studied in the electron and polarizing microscopes and by the low-angle scattering of x-rays. The outer walls of these cells are composed of multiple layers of cellulose microfibrils oriented longitudinally; initially the number of layers is between 10 and 15 but this increases to about 25 in older tissue. Where epidermal cells touch, these multiple layers fuse gradually into a primary wall of the normal type between cells. In these radial walls, the microfibrils are oriented transversely. Possible mechanisms for the growth of the multilayered outer wall during cell elongation are discussed.     

Light-induced inhibition of elongation growth in sunflower hypocotyls

Summary The long-term effects of white light (WL) on epidermal cell elongation and the mechanical properties and ultrastructure of cell walls were investigated in the subapical regions of hypocotyls of sunflower seedlings (Helianthus annuus L.) that were grown in darkness. Upon transition to WL a drastic inhibition of epidermal cell elongation was observed. However, the mechanical properties of the inner tissues (cortex, vascular bundles, and pith) were unaffected by WL. Thus, the light-induced decrease in cell wall plasticity measured on entire stems occurs exclusively in the peripheral tissues (epidermis and 2 to 3 subepidermal cell layers).An electronmicroscopic investigation of the epidermal cell walls showed that they are of the helicoidal type with the direction of microfibrils monotonously changing during deposition. This cell wall type was identified by the appearance of arced patterns of microfibrils in cell walls sectioned oblique to the plane of their synthesis. WL irradiation did not change the periodicity of this pattern nor the thickness of the lamellae. Thus, the inhibition of cell elongation was not caused or accompanied by a shift in the direction of microfibril deposition in the growth-limiting outer tissues. However, cell wall thickness, the number of lamellae and hence the amount of cellulose oriented parallel and transverse to the longitudinal cell axis increased in WL. This may account for the effect of WL on the reduction of cell wall plasticity and growth.Abbreviations D darkness - PATAg periodic acid-thiocarbohydracide-silver protein - WL white light     

Development of the endosperm in rice (Oryza sativa L.): Cellularization

The syncytial endosperm of rice undergoes cellularization according to a regular morphogenetic plan. At 3 days after pollination (dap) mitosis in the peripheral synctium ceases. Radial systems of microtubules emanating from interphase nuclei define nuclear-cytoplasmic domains (NCDs) which develop axes perpendicular, to the embryo sac wall. Free-growing anticlinal walls between adjacent NCDs compart-mentalize the cytoplasm into open-ended alveoli which are overtopped by syncytial cytoplasm adjacent to the central vacuole. At 4 dap, mitosis resumes as a wave originating adjacent to the vascular bundle. The spindles are oriented parallel to the alveolar walls and cell plates formed in association with interzonal phragmoplasts result in periclinal walls that cut off a peripheral layer of cells and an inner layer of alveoli displaced toward the center. Polarized growth of the newly formed alveoli and elongation of the anticlinal walls occurs during interphase. The next wave of cell division in the alveoli proceeds as the first and a second cylinder of cells is cut off inside the peripheral layer. The periods of polarized growth/anticlinal wall elongation alternating with periclinal cell division are repeated 3–4 times until the grain is filled by 5 dap.     

The primary walls of cotton fibers contain an ensheathing pectin layer

Summary Cotton fiber walls (1–2 days post anthesis) are distinctly bilayered compared to those of nonfiber epidermal cells, with a more electron-opaque outer layer and a less electron-opaque, more finely fibrillar inner layer. When probed with antibodies and affinity probes to various saccharides, xyloglucans and cellulose are found exclusively in the inner layer and de-esterified pectins and extensin exclusively in the outer layer. Ovular epidermal cells that do not differentiate into fibers have no pectin sheath, but are labelled throughout with antixyloglucan and cellulase-gold probes. Middle lamellae between adjacent cells were clearly labelled with the antibodies to de-esterified pectins, however. Similarly, cell walls of leaf trichomes have a bilayered wall strongly enriched in pectin, whereas other epidermal cells are not bilayered and are pectin poor. These data indicate that one of the early markers of fiber and trichome cells from other epidermal cells involves the production of a pectin layer. The de-esterified pectins present in the ensheathing layer may allow for expansion and elongation of the fiber cells that does not occur in the other epidermal cells without such a sheath or may even be a consequence of the elongation process.     

Cell-wall synthesis and elongation growth in hypocotyls of Helianthus annuus L.

The relationship between growth and increase in cell-wall material (wall synthesis) was investigated in hypocotyls of sunflower seedlings (Helianthus annuus L.) that were either grown in the dark or irradiated with continuous white light (WL). The peripheral three to four cell layers comprised 30–50% of the entire wall material of the hypocotyl. The increase in wall material during growth in the dark and WL, respectively, was larger in the inner tissues than in the peripheral cell layers. The wall mass per length decreased continuously, indicating that wall thinning occurs during growth of the hypocotyl. When dark-grown seedlings were transfered to WL, a 70% inhibition of growth was observed, but the increase in wall mass was unaffected. Likewise, the composition of the cell walls (cellulose, hemicellulose, pectic substances) was not affected by WL irradiation. Upon transfer of dark-grown seedlings into WL a drastic increase in wall thickness and a concomitant decrease in cell-wall plasticity was measured. The results indicate that cell-wall synthesis and cell elongation are independent processes and that, as a result, WL irradiation of etiolated hypocotyls leads to a thickening and mechanical stiffening of the cell walls.     

Differential Effect of Auxin on Molecular Weight Distributions of Xyloglucans in Cell Walls of Outer and Inner Tissues from Segments of Dark Grown Squash (Cucurbita maxima Duch.) Hypocotyls

Effects of indole-3-acetic acid (IAA) on the mechanical properties of cell walls and structures of cell wall polysaccharides in outer and inner tissues of segments of dark grown squash (Cucurbita maxima Duch.) hypocotyls were investigated. IAA induced the elongation of unpeeled, intact segments, but had no effect on the elongation of peeled segments. IAA induced the cell wall loosening in outer tissues as studied by the stress-relaxation analysis but not in inner tissues. IAA-induced changes in the net sugar content of cell wall fractions in outer and inner tissues were very small. Extracted hemicellulosic xyloglucans derived from outer tissues had a molecular weight about two times as large as in inner tissues, and the molecular weight of xyloglucans in both outer and inner tissues decreased during incubation. IAA substantially accelerated the depolymerization of xyloglucans in outer tissues, while it prevented that in inner tissues. These results suggest that IAA-induced growth in intact segments is due to the cell wall loosening in outer tissues, and that IAA-accelerated depolymerization of hemicellulosic xyloglucans in outer tissues is involved in the cell wall loosening processes.     

Cellulose orientation at the surface of the Arabidopsis seedling. Implications for the biomechanics in plant development

In diffuse growing cells the orientation of cellulose fibrils determines mechanical anisotropy in the cell wall and hence also the direction of plant and organ growth. This paper reports on the mean or net orientation of cellulose fibrils in the outer epidermal wall of the whole Arabidopsis plant. This outer epidermal wall is considered as the growth-limiting boundary between plant and environment. In the root a net transverse orientation of the cellulose fibrils occurs in the elongation zone, while net random and longitudinal orientations are found in subsequent older parts of the differentiation zone. The position and the size of the transverse zone is related with root growth rate. In the shoot the net orientation of cellulose fibrils is transverse in the elongating apical part of the hypocotyl, and longitudinal in the fully elongated basal part. Leaf primordia and very young leaves have a transverse orientation. Throughout further development the leaf epidermis builds a very complex pattern of cells with a random orientation and cells with a transverse or a longitudinal orientation of the cellulose fibrils. The patterns of net cellulose orientation correlate well with the cylindrical growth of roots and shoots and with the typical planar growth of the leaf blade. On both the shoot and the root surface very specific patterns of cellulose orientation occur at sites of specific cell differentiation: trichome-socket cells complexes on the shoot and root hairs on the root.     

Cooperation of epidermis and inner tissues in auxin-mediated growth of maize coleoptiles

The function of the epidermis in auxinmediated elongation growth of maize (Zea mays L.) coleoptile segments was investigated. The following results were obtained: i) In the intact organ, there is a strong tissue tension produced by the expanding force of the inner tissues which is balanced by the contracting force of the outer epidermal wall. The compression imposed by the stretched outer epidermal wall upon the inner tissues gives rise to a wall-pressure difference which can be transformed into a water-potential difference between inner tissues and external medium (water) by removal of the outer epidermal wall. ii) Peeled segments fail to respond to auxin with normal growth. The plastic extensibility of the inner-tissue cell walls (measured with a constant-load extensiometer using living segments) is not influenced by auxin (or abscisic acid) in peeled or nonpeeled segments. It is concluded that auxin induces (and abscisic acid inhibits) elongation of the intact segment by increasing (decreasing) the extensibility specifically in the outer epidermal wall. In addition, tissue tension (and therewith the pressure acting on the outer epidermal wall) is maintained at a constant level over several hours of auxin-mediated growth, indicating that the inner cells also contribute actively to organ elongation. However, this contribution does not involve an increase of cell-wall extensibility, but a continuous shifting of the potential extension threshold (i.e., the length to which the inner tissues would extend by water uptake after peeling) ahead of the actual segment length. Thus, steady growth involves the coordinated action of wall loosening in the epidermis and regeneration of tissue tension by the inner tissues. iii) Electron micrographs show the accumulation of striking osmiophilic material (particles of approx. 0.3 m diameter) specifically at the plasma membrane/cell-wall interface of the outer epidermal wall of auxin-treated segments. iv) Peeled segments fail to respond to auxin with proton excretion. This is in contrast to fusicoccin-induced proton excretion and growth which can also be readily demonstrated in the absence of the epidermis. However, peeled and nonpeeled segments show the same sensitivity to protons with regard to the induction of acid-mediated in-vivo elongation and cell-wall extensibility. The observed threshold at pH 4.5–5.0 is too low to be compatible with a second messenger function of protons also in the growth response of the inner tissues. Organ growth is described in terms of a physical model which takes into account tissue tension and extensibility of the outer epidermal wall as the decisive growth parameters. This model states that the wall pressure increment, produced by tissue tension in the outer epidermal wall, rather than the pressure acting on the inner-tissue walls, is the driving force of growth.Abbreviations and symbols E _el, E _pl elastic and plastic in-vitro cell-wall extensibility, respectively - E _tot E _el+E _pl - FC fusicoccin - IAA indole-3-acetic acid - IT inner tissue - ITW inner-tissue walls - OEW outer epidermal wall - osmotic pressure - P wall pressure - water potential     

Role of the outer tissue in abscisic acid-mediated growth suppression of etiolated squash hypocotyl segments

Exogenously applied abscisic acid (ABA) substantially suppressed the elongation of hypocotyl segments of etiolated squash ( Cucurbita maxima Duch. cv. Houkou-Aokawaamaguri) after a 3 h lag period, without changes in the osmolalities of the apoplastic and symplastic solutions in the segment.
Segments with the outer tissues removed elongated more rapidly than unpeeled segments (whole segments). ABA did not suppress the elongation of peeled segments. When the segments were incubated in [¹⁴C]-glucose, radioactivity was more effectively incorporated into the cell wall fractions of the outer than into those of the inner tissue. ABA significantly inhibited the incorporation of radioactivity into hermicellulose and cellulose of the outer tissue prior to the suppression of segment elongation, but it did not inhibit the incorporation into the pectic traction of the outer tissue or into any of the cell wall fractions of the inner tissue. These results indicate that ABA primarily affected the outer tissue, in which it specifically reduced the synthesis of hemicellulose and cellulose prior to the ABA-mediated suppression of growth.     

Deposition of glucuronoxylans on the secondary cell wall of Japanese beech as observed by immuno-scanning electron microscopy

Summary Glucuronoxylans (GXs), the main hemicellulosic component of hardwoods, are localized exclusively in the secondary wall of Japanese beech and gradually increase during the course of fiber differentiation. To reveal where GXs deposit within secondary wall and how they affect cell wall ultrastructure, immuno-scanning electron microscopy using anti-GXs antiserum was applied in this study. In fibers forming the outer layer of the secondary wall (S₁), cellulose fibrils were small in diameter and deposited sparsely on the inner surface of the cell wall. Fine fibrils with approximately 5 nm width aggregated and formed thick fibrils with 12 nm width. Some of these thick fibrils further aggregated to form bundles which labelled positively for GXs. In fibers forming the middle layer of the secondary wall (S₂), fibrils were thicker than those found in S₁ forming fibers and were densely deposited. The S₂ layer labelled intensely for GXs with no preferential distribution recognized. Compared with newly formed secondary walls, previously formed secondary walls were composed of thick and highly packed microfibrils. Labels against GXs were much more prevalent on mature secondary walls than on newly deposited secondary walls. This result implies that the deposition of GXs into the cell wall may occur continuously after cellulose microfibril deposition and may be responsible for the increase in diameter of the microfibrils.Abbreviations GXs glucuronoxylans - PBS phosphate-buffered saline - RFDE rapid-freeze and deep-etching technique - FE-SEM field emission scanning electron microscope - TEM transmission electron microscope     

Seed Epidermis Development and Histochemistry in Solanum melongena L. and S. violaceum Ort.

Structure, development and histochemistry of the seed epidermiswere studied inSolanum melongena L. andS. violaceum Ort. usinglight and scanning electron microscopy. The epidermal cellsat the endosperm mother cell stage of ovule development hadthickened outer periclinal walls, consisting of two layers,a thin inner layer, and a thick outer layer. The latter whichstained positively for pectic substances became further thickenedduring the course of seed development; more so inS. melongena.The inner layer of the outer periclinal wall also was thickenedby depositions of cellulose but remained comparatively thin.The development of the inner periclinal and anticlinal wallstook place by the uneven deposition of concentric layers. Thesesecondary wall thickenings which appeared as pyramids in transversesection stained for cellulose, lignin and pectin. Further unevensecondary thickenings near the outer part of the anticlinalwalls resulted in the formation of projections which were hair-or ribbon-like in appearance. InS. melongena, these projectionsprogressed only a short distance from the anticlinal wall. InS.violaceum, on the other hand, they grew much longer formingstriations on the inside of the outer periclinal wall. InS.melongena, partial removal of the outer periclinal wall by enzymeetching exposed to surface view a beaded appearance of the cellboundaries. Complete erosion of the outer periclinal wall revealedthe hair-like projections of the underlying anticlinal walls.InS. violaceum, enzyme treatment exposed the striations whichformed bridge-like structures over the curves in the anticlinalwalls. Solanum melongena ; Solanum violaceum; seed epidermis; seed structure; seed development; cell wall histochemistry; cell wall projections; cell wall striations     

Structure of the endosperm in the mature seed of Melilotus alba]

The results of the exam at the light, the fluorescence and the scanning electron microscope of the endosperm of Melilotus alba mature impermeable seeds are reported. Cryostat sections, semithin sections and squashes are observed. Melilotus alba endosperm is variable in thickness and envelopes cotyledons and radicle. Its "aleurone" layer is one-cell thick, while the number of layers of its internal cells varies in relation to the location in the seed. In the aleurone cells, the cytoplasm and the outer portion of the wall are autofluorescent; tannic acid-ferric chloride stains the outer portion of the wall and allows to see clearly the inner thickenings, DAPI and haematoxylin demonstrate the presence of the nucleus. The cytoplasm of these cells is coloured by Sudan black b, and its fluorescence is enhanced by auramine and calcofluor white. Calcofluor white enhances the fluorescence of the outer portion of these walls, too, but is without effect on the non-autofluorescent thickening, indicating presence of cellulose only in the first case. Callose is absent. Also the thin autofluorescent walls of the endosperm inner cells react positively to calcofluor. These cells are very large, almost completely filled with "gelatinous" substances--the galactomannans--and very rarely contain a nucleus.     

Cell-wall structure and anisotropy in procuste, a cellulose synthase mutant of Arabidopsis thaliana

In dark-grown hypocotyls of the Arabidopsis procuste mutant, a mutation in the CesA6 gene encoding a cellulose synthase reduces cellulose synthesis and severely inhibits elongation growth. Previous studies had left it uncertain why growth was inhibited, because cellulose synthesis was affected before, not during, the main phase of elongation. We characterised the quantity, structure and orientation of the cellulose remaining in the walls of affected cells. Solid-state NMR spectroscopy and infrared microscopy showed that the residual cellulose did not differ in structure from that of the wild type, but the cellulose content of the prc-1 cell walls was reduced by 28%. The total mass of cell-wall polymers per hypocotyl was reduced in prc-1 by about 20%. Therefore, the fourfold inhibition of elongation growth in prc-1 does not result from aberrant cellulose structure, nor from uniform reduction in the dimensions of the cell-wall network due to reduced cellulose or cell-wall mass. Cellulose orientation was quantified by two quantitative methods. First, the orientation of newly synthesised microfibrils was measured in field-emission scanning electron micrographs of the cytoplasmic face of the inner epidermal cell wall. The ordered transverse orientation of microfibrils at the inner face of the cell wall was severely disrupted in prc-1 hypocotyls, particularly in the early growth phase. Second, cellulose orientation distributions across the whole cell-wall thickness, measured by polarised infrared microscopy, were much broader. Analysis of the microfibril orientations according to the theory of composite materials showed that during the initial growth phase, their anisotropy at the plasma membrane was sufficient to explain the anisotropy of subsequent growth.     

Structure of mucilaginous epidermal cell walls in Passerina (Thymelaeaceae)

Leaves of Passerina are inversely ericoid. Adaxial epidermal cells are relatively small; abaxial ones are large and tanniniferous. Mucilaginous epidermal cells are usually present in many Thymelaeaceae, including Passerina , mainly in the abaxial epidermis. They are unequally divided by a periclinal wall-like septum into two separate compartments: (1) the outer, adjacent to the cuticle, containing mostly tanniniferous substances and (2) the inner, containing mucilage. This type of epidermis has often been incorrecdy described as uni-, bi- or multiseriate. Transmission electron microscopy revealed mucilage, characterized by microfibrils, embedded between die innermost wall-like septum and outermost layers of the inner periclinal cell wall. As accumulation of mucilage increases, the innermost (adjacent to the cell contents) layer of the original periclinal cell wall is pressed against the cytoplasm, thus forming a clearly demarcated cellulose periclinal wall which divides the epidermis cell into two compartments, the inner wiuh mucilage and the outer comprising the cell lumen. Existing controversy is critically discussed. Our observations confirm the authenticity of mucilagination in epidermal cell walls.     

Changes in cell-wall polysaccharide composition of developingNitella internodes

Changes in the uronide, neutral-polysacharide, and cellulose composition of the cell wall ofNitella axillaris Braun were followed throughout development of the internodes and correlated with changes in growth rate. As the cells increased in length from 4 to 80 mm during development, the relative growth rate decreased. Cell wall thickness, as measured by wall density, increased in direct proportion to diameter, indicating that cell-wall stress did not change during elogation. Cell-wall analyses were adapted to allow determination of the composition of the wall of single cells. The total amounts of uronides, neutral sugars and cellulose all increased during development. However, as the growth rate decreased, the relative proportions of uronides and neutral sugars, expressed as percent of the dry weight of the wall, decreased, while the proportion of cellulose increased. The neutral sugars liberated upon hydrolysis ofNitella walls are qualitatively similar to those found in hydrolysates of higher plant cell walls: glucose, xylose, mannose, galactose, arabinose fucose and rhamnose. Only the percentage of galactose was found to increase in walls of mature cells, while the percentage of all other sugars decreased. The rate of apposition (g of wall material deposited per unit wall surface area per hour) of neutral polysaccharides decreased rapidly with decreasing growth rate during the early stages of development. The rate of apposition of uronides decreased more steadily throughout development, while that of cellulose, after an early decline, remained constant until dropping off at the end of the elongation period. These correlations between decreasing growth rate and decreasing rate of apposition of neutral sugars and uronides indicate that synthesis of these cell-wall components could be involved in the regulation of the rate of cell elongation inNitella.