Outer Epidermal Wall Research Articles

The epidermal surface of the maize root tip: III. Isolation of the surface and characterization of some of its structural and mechanical properties.

The surface of the young epidermal cells of maize roots is composed o) three layers; the inner layer (LI), which is the outer epidermal Wall, overlaid by a pellicle consisting of a thick, coherent inner layer (L2) and a very thin, loosely organized outer layer (L3). The entire surface can be removed intact to produce either narrow, circumferential strips or apical halftones, by gently prying it loose in the circumferential direction by hand with insect pins. Usually only short remnants of anticlinical walls of the epidermal cells remain attached. These isolated surface pieces always curl outward at the free circumferential edges in the longitudinal direction of the original intact root. When the strips are deliberated stretched alone their long axis (i.e., the original circumferential direction) they elongate irreversibly by us much us two thirds of their length, before showing some elastic deformation and breaking. Some plastic deformation may occur in the original longitudinal direction of the root during removal of the strips. The plastic deformation opens the helicoidal array of microfibrils in the L1 layer. Deformation also produces structural changes over the original radial walls and those transverse anticlinal walls that form boundaries of cell packets derived from single cells. In these positions the L1 layers over adjacent cells separate in the direction of the applied stress. This occurs by the separation of the L I layers of adjacent cells and the stretching of the inward projection of the amorphous L2 layer of the pellicle which lies; over these original anticlinal walls. There is much less or no separation of the L1 layers over anticlinal walls Of adjoining daughter cells in the epidermis. The pellicle always remains firmly attached to the outer epidermal wall during removal of the surface strips. On removal, these strips shorten in their original longitudinal direction in situ, indicating a release of tension imposed by underlying cells. Their outward curling suggests stress between the wall and pellicle of the outer epidermal surface in the intact root. These findings focus attention on structural differences between sites where anticlinal walls of different origin join the outer surface, and on possible differences in surface extensibility at each sites.

Contributions of the surface of the root tip to the growth of Zea roots in soil

The development of the epidermal layer of roots of Zea is traced from the quiescent centre to the zone where root hairs develop. In the zone of cell division a three layered coat forms on the outside of the epidermal cells consisting of the outer epidermal walls, overlaid by a two-layered pellicle composed of a thick fibrillar inner layer of polysaccharide, and a thin fibrillar outer layer of protein. The epidermal cells divide several times in the same longitudinal file but rarely across a radius to give a new longitudinal file. Thus, the radial walls become much thicker than all but the original transverse walls, and packets of up to 32 daughter cells derived from a single initial may be distinguished. The pellicle develops during these divisions as a continuum over the outer walls of the daughter cells. It is proposed that the pellicle provides a stiffening to the forward end of the root which permits it to penetrate soil without bending. Support for this hypothesis is shown by the Zea mays mutant Ageotropic in which the pellicle is absent, the epidermal surface is disorganized, and which grows crookedly through soil. In the zone of extension growth of normal roots of two Zea species the pellicle thins and disappears. Circumferential strips of the pellicle were peeled off the young epidermal cells and could be stretched to twice their length. This deformation is partly the result of the pellicle stretching and breaking above the attachments of the radial walls. After normal thinning of the pellicle, detachment of the radial walls at their outer ends produces a corrugated surface in the proximal zone of the root tips. In dicotyledons (e.g., soybean), there is no similar pellicle, but a stiff root tip is produced by a long multi-layered root cap, the proximal portion of which covers the elongating epidermal surface.

The current status of the acid‐growth hypothesis

SUMMARYExcised coleoptile segments that were depleted of endogenous auxin after pre‐incubation in water respond to the phytotoxin fusicoccin (FC) and the plant hormone auxin (indole‐3‐acetic acid, IAA) with an enhancement of growth. This promotion of organ elongation is caused by an increase in the extensibility of the thick, growth‐limiting outer epidermal wall. The acid‐growth hypothesis postulates that both FC and IAA cause wall‐loosening and the concomitant induction of growth by rapid acidification of the extension‐limiting cell wall. At saturating concentrations FC and IAA cause very similar growth responses. However, the equilibrium‐pH in the incubation medium (∼ pH of the peripheral cell wall) of abraded FC‐treated segments is 3.5–4.0, whereas in the presence of IAA a pH of 4.8–5.0 is established. The FC‐ and IAA‐mediated induction of growth can be stimulated by an external buffer of pH 3.5–4.0. Acid buffers of pH 5, however, cause almost no enhancement of growth compared WJth the water‐control. FC‐mediated growth can be inhibited by neutral buffers infiltrated into the outer epidermal wall, but a substantial growth response occurs after addition of IAA under identical conditions. A sub‐optimal concentration of FC was used to mimic the effect of IAA on acid secretion. When a pH of 4.8–5.0 was established by FC in the peripheral wall no promotion of growth occurred. Hence, IAA‐induced proton excretion is insufficient to cause cell‐wall loosening in the coleoptile. Abraded segments that rapidly elongate in acid buffers (pH 3.5–4.0) respond to IAA with an additional enhancement of growth, but under identical conditions FC is without effect. The growth‐promoting effects of acid and IAA are additive, i.e. IAA and acid act via separate mechanisms. These results are consistent with the acid‐growth hypothesis of FC action. However, they indicate that IAA induces growth by a mechanism independent of cell‐wall acidification. Alternative hypotheses of IAA‐mediated cell‐wall loosening are discussed. Contents Summary 549 I. Introduction 550 II. Segment elongation and the organismal concept of multicellularity 550 III. FC‐ and IAA‐mediated acid secretion 552 IV. The acid‐growth hypothesis of FC action 555 V. The acid‐growth hypothesis of IAA action 560 VI. Conclusions 567 Acknowledgements 568

Reorientation of microtubules at the outer epidermal wall of maize coleoptiles by phytochrome, blue-light photoreceptor, and auxin

The effects of red and blue light on the orientation of cortical microtubules (MTs) underneath the outer epidermal wall of maize (Zea mays L.) coleoptiles were investigated with immunofluorescent techniques. The epidermal cells of dark-grown coleoptiles demonstrated an irregular pattern of regions of parallel MTs with a random distribution of orientations. This pattern could be changed into a uniformly transverse MT alignment with respect to the long cell axis by 1 h of irradiation with red light. This response was transient as the MTs spontaneously shifted into a longitudinal orientation after 1–2 h of continued irradiation. Induction/reversion experiments with short red and far-red light pulses demonstrated the involvement of phytochrome in this response. In contrast to red light, irradiation with blue light induced a stable longitudinal MT alignment which was established within 10 min. The blue-light response could not be affected by subsequent irradiations with red or far-red light indicating the involvement of a separate blue-light photoreceptor which antagonizes the effect of phytochrome. In mixed light treatments with red and blue light, the blue-light photoreceptor always dominated over phytochrome which exhibited an apparently less stable influence on MT orientation. Long-term irradiations with red or blue light up to 6 h did not reveal any rhythmic changes of MT orientation that could be related to the rhythmicity of helicoidal cell-wall structure. Subapical segments isolated from dark-grown coleoptiles maintained a longitudinal MT arrangement even in red light indicating that the responsiveness to phytochrome was lost upon isolation. Conversely auxin induced a transverse MT arrangement in isolated segments even in blue light, indicating that the responsiveness to blue-light photoreceptor was eliminated by the hormone. These complex interactions are discussed in the context of current hypotheses on the functional significance of MT reorientations for cell development.

Changes in composition of the outer epidermal cell wall of pea stems during auxin-induced growth

The gross composition of the outer epidermal cell wall from third internodes of Pisum sativum L. cv. Alaska grown in dim red light, and the effect of auxin on that composition, was investigated using interference microscopy. Pea outer epidermal walls contain as much cellulose as typical secondary walls, but the proportion of pectin to hemicellulose resembles that found in primary walls. The pectin and hemicellulose fractions from epidermal peels, which are enriched for outer epidermal wall but contain internal tissue as well, are composed of a much higher percentage of glucose and glucose-related sugars than has been found previously for pea primary walls, similar to non-cellulosic carbohydrate fractions of secondary walls. The epidermal outer wall thus has a composition rather like that of secondary walls, while still being capable of elongation. Auxin induces a massive breakdown of hemicellulose in the outer epidermal wall; nearly half the hemicellulose present is lost during 4 h of growth in the absence of exogenous sugar. The percentage breakdown is much greater than has been seen previously for whole pea stems. It has been proposed that a breakdown of xyloglucan could be the basis for the mechanical loosening of the outer wall. This study provides the first evidence that such a breakdown could be occurring in the outer wall.

Total epidermal cell walls of pea stems respond differently to auxin than does the outer epidermal wall alone

The effect of auxin on cell wall mass in the epidermis of third internodes of Pisum sativum L. cv. Alaska grown in dim red light was investigated using epidermal peels, to determine whether epidermal peels reflect the behavior of the outer epidermal cell wall. In contrast to the outer epidermal wall itself, where auxin caused thinning in proportion to growth (M.S. Bret-Harte et al, 1991, Planta 185, 462–471), auxin promoted an increase in wall mass in epidermal peels from treated internode segments in the absence of exogenously supplied sugar. The percentage gain in mass was smaller than the percentage elongation, however, so mass per unit length decreased in peels from auxin-treated segments. Epidermal peels from auxin-treated segments gained more wall mass than control peels even when adhering internal tissue at the basal end of the peel was removed. Epidermal peels also had a gross composition different from that of the outer wall alone (M.S. Bret-Harte and L.D. Talbott, 1993, Planta 190, 369–378). These discrepancies can be explained by the observation that the outer wall makes up only 30% of the mass of the epidermal peel. It appears that the inner walls of the epidermis, and walls of the outer layer of cortical cells that remain attached to the epidermis during peeling, nearly maintain their thickness by biosynthesis while the outer wall loses mass as previously described (Bret-Harte et al. 1991). These results indicate that epidermal peels may not be a good system for examining the biochemical and physiological properties of the outer epidermal cell wall.

Analysis of the Growth Response of Air‐Grown Rice Coleoptiles to Submergence

AbstractThe effect of submergence of air‐grown rice seedlings (Oryza sativa L. var. Sasanishiki) on coleoptile growth and ultrastructure, extensibility and chemical composition of the cell walls was investigated. The lag‐time between start of submergence and the onset of the enhancement of growth was less than 4 h. The growth response was associated with a drastic thinning of the cell walls and an increase in wall extensibility. At the outer epidermal wall of both air‐grown and submerged coleoptiles electron‐dense (osmiophilic) particles were detected. During submergence, the net accumulation of cellulose and hemicellulose was reduced, but the increase in pectic substances was unaffected. Submergence caused an 80% inhibition of the net accumulation of wall‐bound phenolics (ferulic‐ and diferulic acid) compared with air‐grown controls. The osmotic concentration of the tissue saps was not affected by submergence. Our results support the hypothesis that rapid coleoptile elongation under water is caused by an inhibition of the formation of phenolic cross‐links between matrix polysaccharides via diferulate, which results in a mechanical stiffening of the cell walls in the air‐grown coleoptile.

Anatomy of fruit of buttercup squash(Cucurbita maximaD.) surface, cuticle, and epidermis

Abstract The surface morphology, and cuticle and outer epidermal cell wall anatomy of developing squash fruit (Cucurbita maxima Duch. hybrid ‘Delica') were examined using light microscopy, and scanning and transmission electron microscopy. During the period from flowering to maturity, fruit surface morphology changed from epidermal cells with raised dome‐shaped outer periclinal walls, inconspicuous smooth wax and three trichome types, to a surface with depressions and ridges encrusted with waxy deposits and devoid of trichomes. The thicknesses of both cuticle and cell wall increased to maturity. The four distinct areas of the fruit surface— green, white, rib, and ground‐spot—had similar cell wall and cuticle thickness at maturity, although cuticularisation of anticlinal walls was less developed in the ground spot area. None of the areas observed showed special features that might predispose the surface to fungal infection.

Histological examination of the interaction between Rhynchosporium secalis and susceptible and resistant cultivars of barley

The infection processes of Rhynchosporium secalis were compared in three barley cultivars, Lenka (susceptible), Digger (resistant), and Osiris (highly resistant). Appressorial formation and attempted cuticular penetrations were similar in cvs Lenka and Digger. Penetration attempts were associated with the formation of papillae and haloes in the cell walls around the penetration pegs in all cultivars, but the papilla size increased with time after inoculation and became significantly larger in resistant than in susceptible plants. A correlation, although less pronounced, was found between halo size and plant resistance. After successful penetration of the cuticle in Lenka, first thin hyphae, then later thicker hyphae developed; both developed subcuticularly. The thick hyphae proceeded into the outer periclinal epidermal cell walls, through the anticlinal walls and into the inner periclinal walls of the epidermis. Very few hyphae grew into the intercellular spaces of the mesophyll. As infection proceeded, the cells collapsed in advance of hyphae, staining affinity of chloroplasts changed, and polyphenolic substances accumulated. Conidia were produced on short conidiophores penetrating the otherwise intact cuticle. In cv Digger, a strong correlation was found between papilla size and inhibition of fungal penetration, indicating papilla formation as a resistance mechanism. Only a few hyphae were established subcuticularly, no cell collapse or accumulation of polyphenolic substances took place, and only restricted sporulation occurred in the leaf margins. The only macroscopic symptom of infection was limited chlorosis.

Cell-wall tension of the inner tissues of the maize coleoptile and its potential contribution to auxin-mediated organ growth

Plant organs such as maize (Zea mays L.) coleoptiles are characterized by longitudinal tissue tension, i.e. bulk turgor pressure produces unequal amounts of cell-wall tension in the epidermis (essentially the outer epidermal wall) and in the inner tissues. The fractional amount of turgor borne by the epidermal wall of turgid maize coleoptile segments was indirectly estimated by determining the water potential ψ(*) of an external medium which is needed to replace quantitatively the compressive force of the epidermal wall on the inner tissues. The fractional amount of turgor borne by the walls of the inner tissues was estimated from the difference between -ψ(*) and the osmotic pressure of the cell sap (πi) which was assumed to represent the turgor of the fully turgid tissue. In segments incubated in water for 1 h, -ψ(*) was 6.1-6.5 bar at a πi of 6.7 bar. Both -ψ(*) and πi decreased during auxin-induced growth because of water uptake, but did not deviate significantly from each other. It is concluded that the turgor fraction utilized for the elastic extension of the inner tissue walls is less than 1 bar, i.e. less than 15% of bulk turgor, and that more than 85% of bulk turgor is utilized for counteracting the high compressive force of the outer epidermal wall which, in this way, is enabled to mechanically control elongation growth of the organ. This situation is maintained during auxin-induced growth.

Leaf anatomical studies of Eugenia L. (Myrtaceae) species from the Malay Peninsula

KHATIJAH, H. H., CUTLER, D. F. & MOORE, D. M., 1992. Leaf anatomical studies of Eugenia L. species from the Malay Peninsula. The leaf and petiole anatomy of 25 Malaysian species of Eugenia L. sensu lato was investigated to determine the value of vegetative anatomy for the identification of non-flowering material. Twelve characters proved useful for the identification of species: type of stomata; shape of leaves in T.S.; shape of midrib bundle; sclerenchyma sheath; cutinization of outer epidermal wall; presence or absence of sclereids, idioblasts, hypodermis, columnar epidermal cells, solitary crystals; number of palisade layers and the shape of vascular strand, sclerenchyma sheath and sclereids in the parenchyma of the petiole. Using these characters an anatomical key was constructed to assist identification of the species. Only the closely related species E. stapfiana and E. nilidula could not be separated on anatomical characters. The anatomical data support the treatment of Henderson (1949) of section Acmena but not of sections Cleistocalyx and Fissicalyx, which fit better in section Syzygium.

Ultrastructure and development of the outer epidermal wall of spruce (Picea abies) needles

The outer epidermal wall of Picea abies needles was investigated throughout its life-span. Several cytochemical procedures at the light and electron microscope level including enzyme–gold affinity labeling and elemental mapping by electron energy loss spectroscopy were applied to gain detailed information about its fine structure. The adult outer epidermal wall averages 9 μm in thickness and occupies half the cell height. It is composed of five main layers and additional epicuticular waxes. In the cuticular membrane a thin homogeneous cuticle proper is clearly distinguishable from the cuticular layer, which is composed of an exterior striated, a middle reticulate, and an interior arborescent sublayer. Cellulose and mannans were localized and identified among other polysaccharides in the cuticular layer in situ. Arborescent structures extend to an adjacent, usually lignified layer, containing numerous calcium crystals. Beneath this is an electron-dense layer, formed mainly of hemicelluloses. The innermost wall layer comprises several alternately arranged lamellae formed of several types of polysaccharides and differing mainly in their polyphenol content. During ontogeny of the outer epidermal wall, lasting a whole year, five phases were distinguished: a protodermal phase, one of even growth, one of uneven growth, and one of lignification and further growth. With the fifth phase the outer epidermal wall of current needles achieves maturity in late autumn. No differences in ultrastructure and development of the outer epidermal wall between apparently healthy and damaged needles were observed. Key words: Picea abies, outer epidermal wall, light and electron microscopical cytochemistry, enzyme–gold technique, polysaccharides, calcium crystals.

Physical extensibility of maize coleoptile cell walls: apparent plastic extensibility is due to elastic hysteresis

Segments of maize (Zea mays L.) coleoptiles demonstrate plastic cell-wall extensibility (Epl) as operationally defined by the amount of irreversible strain elicited by stretching living or frozen-thawed tissue under constant load in an extensiometer (creep test). Changes of Epl are correlated with auxin- and abscisic-acid-dependent growth responses and have therefore been causally related to hormone-controlled cell-wall loosening. Auxin induces an increase of Epl specifically in the outer epidermal wall of maize coleoptiles which is considered as the growth-limiting wall of the organ. However, detailed kinetic measurements of load-induced extension of frozen-thawed coleoptile segments necessitates a revision of the view that Epl represents a true plastic (irreversible) wall deformation. Segments demonstrate no significant irreversible extension when completely unloaded between loading cycles. Moreover, Epl can be demonstrated repeatedly if the same segment is subjected to repeated loading cycles in the extensiometer. It is shown that these phenomena result from the hysteresis behaviour of the cell wall. Stress-strain curves for loading and unloading form a closed hysteresis loop, the width of which represents Epl at a particular load. Auxin-treatment of segments leads to a deformation of the hysteresis loop, thereby giving rise to an increase of Epl. These results show that the creep test estimates the viscoelastic (retarded elastic) properties rather than the plastic properties of the wall.

The Hormonal Regulation of Cell Elongation Through Changes in Microtubule Orientation

Previous articleNext article No AccessThe Hormonal Regulation of Cell Elongation Through Changes in Microtubule OrientationRobert W. SeagullRobert W. Seagull Search for more articles by this author PDFPDF PLUS Add to favoritesDownload CitationTrack CitationsPermissionsReprints Share onFacebookTwitterLinkedInRedditEmail SectionsMoreDetailsFiguresReferencesCited by International Journal of Plant Sciences Volume 153, Number 2Jun., 1992 Article DOIhttps://doi.org/10.1086/ijps.153.2.2995636 Views: 4Total views on this site Citations: 4Citations are reported from Crossref Copyright 1992 The University of ChicagoPDF download Crossref reports the following articles citing this article: Robert W. Seagull Cytoskeletal Stability Affects Cotton Fiber Initiation, International Journal of Plant Sciences 159, no.44 (Oct 2015): 590–598.https://doi.org/10.1086/297577Cheryl A. Telmer, W. Newcomb, Daina H. Simmonds Cellular changes during heat shock induction and embryo development of cultured microspores ofBrassica napus cv. Topas, Protoplasma 185, no.1-21-2 (Mar 1995): 106–112.https://doi.org/10.1007/BF01272758K. Zandomeni, P. Schopfer Reorientation of microtubules at the outer epidermal wall of maize coleoptiles by phytochrome, blue-light photoreceptor, and auxin, Protoplasma 173, no.3-43-4 (Sep 1993): 103–112.https://doi.org/10.1007/BF01378999Robert W. Seagull Cytoskeletal involvement in cotton fiber growth and development, Micron 24, no.66 (Jan 1993): 643–660.https://doi.org/10.1016/0968-4328(93)90042-Y

Cytochemical identification of arabinogalactan protein in the outer epidermal wall of maize coleoptiles

Artificial carbohydrate antigen (Yariv reagent), fluorescence-labeled α-L-fucose-binding lectin, and β-D-galactose-binding lectin were used to localize arabinogalactan protein in sections of maize (Zea mays L.) coleoptiles. All three probes bind to cell walls of vascular tissue and the outer epidermis. Intense staining is obtained at the outer and inner faces of the growth-controlling outer epidermal wall. At the inner face of this wall the auxin-inducible "osmiophilic particles", hitherto observed only by electron microscope (Kutschera et al. 1987, Planta 170, 168-180), are strongly stained by all three probes and can therefore be identified as deposits of arabinogalactan protein. It is proposed that this proteoglycan acts as an epidermal wallloosening factor in auxin-mediated coleoptile growth.

Auxin stimulates both deposition and breakdown of material in the pea outer epidermal cell wall, as measured interferometrically

The effect of auxin on the mass per area in the outer epidermal walls of third internodes of Pisum sativum L. cv. Alaska grown in dim red light was investigated using interference microscopy, and rates of net deposition of wall material were calculated. Examination of these net rates under different growth conditions showed that there is no simple relationship between the deposition of mass and growth. Net deposition can be proportional to growth when sufficient substrate for wall synthesis is available, as in intact plants, and in segments treated with indole-3-acetic acid (IAA) plus glucose. Net deposition can cause thickening of the walls when growth is small, as in the case of segments kept without IAA in the presence or absence of glucose, or segments whose growth is inhibited with mannitol. When substrate is limited and growth is large, however, wall expansion can occur with no net deposition, or an actual net loss of wall material can even take place. Auxin appears to induce a breakdown in the walls of segments treated in the absence of glucose, although it promotes synthesis when glucose is present. It is likely that IAA always induces a breakdown of wall material, but that the breakdown is masked when substrate is available for synthesis. Our results indicate that pea epidermal cells have two different auxin-stimulated mechanisms, wall synthesis and wall breakdown, potentially available to loosen their outer epidermal walls to bring about cell enlargement, alternatives which could be employed to different extents depending on substrate conditions.

Cytology of induced systemic resistance of tomato to Phytophthora infestans

The infection of tomato leaves by Phytophthora infestans was followed using cytological methods. Fungal ingress and plant reactions in untreated and induced resistant plants were studied. Systemic disease resistance was induced by a local pre-infection with the same fungus. Induction retarded fungal progress at the leaf surface, epidermis and in the mesophyll. The reduced numbers of germinated cysts indicate the presence of fungitoxic substances on the leaf surface of induced plants. Frequency of fungal penetration through the outer epidermal cell wall was reduced, but only in plants exhibiting a high level of induced resistance. Autofluor-escent material, indicating the presence of lignin-like substances, accumulated rapidly beneath some of the appressoria, but this plant response was similar in induced and non-induced plants. Staining with aniline blue indicated that callose deposition was not involved in induced resistance. Thus, none of the cytologically investigated plant reactions correlated with the reduced penetration frequency observed. In the mesophyll, however, the cytological picture corresponding to a hypersensitive reaction occurred more often in induced plants. It is concluded that reduction of disease severity by induction is the result of the combined action of several successive defence reactions.

The outer epidermal wall structure of African Stapelieae (Asclepiadaceae)

The outer epidermal walls of 65 species incorporated in 14 genera of the tribe Stapelieae were investigated by light and electron microscopy (TEM, SEM). The species, all of them stem succulents preferring the semi‐arid regions of the Palae‐otropics, possess an outer epidermal wall, which is relatively simple in structure. The whole plant body is covered by a massive wax layer, in addition some species bear wax crystals. A cutin layer follows beneath, reticulate over its whole expanse and not divided into a cuticle proper and cutinized layers. A cellulose layer, which often exhibits enclosed cutinbodies, delimits the wall against the plasmalemma.

Microtubule patterning in apical epidermal cells ofVinca minor preceding leaf emergence

Serial thin sectioning for electron microscopy was carried out on the cortical cytoplasm of surface cells of the apical dome ofVinca minor. The cellulose reinforcement pattern in the outer epidermal walls forming this surface is known to correlate well with the decussate phyllotaxis pattern. The purpose of this study was to determine the location of microtubules immediately under these epidermal walls as a first step toward finding out how the cellulose pattern arises. First, correspondence between the patterns of microtubules and cellulose was checked. Second, the role of potential orienting cues for the alignment of microtubule arrays in specific cells was evaluated.

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.