Studies on compression wood—part VII: Distribution of lignin in normal and compression wood of tamarack [Larix laricina (Du Roi) K. Koch]

Summary The distribution of lignin has been studied in tracheids and ray cells of normal and compression wood of tamarack [Larix laricina (Du Roi) K. Koch]. The three layers in the secondary wall of normal wood tracheids are lignified to approximately the same extent, and previous evidence that the S ₃ layer should contain a higher proportion of lignin than the other regions has not been confirmed. The lignin follows closely the orientation of the cellulose microfibrils in all three layers. Compared to the tracheids, the ray cells contain a denser network of lignin in their secondary wall.Only a small proportion of the total lignin in compression wood tracheids is present in the compound middle lamella. The thick S ₁ layer is only slightly lignified; the orientation of the lignin in this region is that of the transversely oriented, lamellated microfibrils. The outer portion of S ₂ consists largely of lignin but also contains lamellae of cellulose microfibrils which probably have the same helical orientation as the microfibrils in the inner part of S ₂. The latter region, which contains the helical cavities, consists of lamellae of cellulose microfibrils which are uniformly encrusted with lignin. The ray cells in compression wood appear to be lignified to the same extent as in normal wood. Transverse sections of the cells reveal a lateral orientation of the lignin. The orientation of the cellulose microfibrils in the S ₂ layer of the first-formed springwood tracheids of compression wood is the same as in the cells which are formed later. It is suggested that for ease of reference, the outer, lignin-rich layer in compression wood tracheids be referred to as the S ₂(L) layer.

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Zusammenfassung Im Druckholz und im normalen Holz von Tamarack (Larix laricina (Du Roi) K. Koch) wurde die Verteilung des Lignins in Tracheiden und Markstrahlzellen untersucht. Die drei Schichten der Sekundärwand in den Tracheiden normalen Holzes werden in nahezu demselben Umfange lignifiziert. Frühere Feststellungen, daß die S ₃-Schicht einen höheren Ligningehalt erreicht als andere Zellwandbereiche, konnten also nicht bestätigt werden. Das Lignin folgt sehr genau der Orientierung der Cellulose-Mikrofibrillen aller drei Schichten. Im Vergleich zu den Tracheiden erfahren die Sekundärwände der Markstrahlzellen eine stärkere Ligninauskleidung.Nur ein geringer Prozentsatz des gesamten Lignins der Druckholztracheiden befindet sich in der Mittellamelle. Die dicke S ₁-Schicht ist nur wenig lignifiziert. Die Orientierung des Lignins in diesem Bereich entspricht den transversal orientierten, lamellierten Mikrofibrillen. Der äußere Teil der S ₂-Schicht enthält sehr viel Lignin, daneben aber auch Lamellen von Cellulose-Mikrofibrillen, die wahrscheinlich dieselbe spiralige Orientierung besitzen wie die Mikrofibrillen des inneren Teiles der S ₂-Schicht. Der letzterwähnte Bereich, der spiralige Kavitäten enthält, weist Lamellen von Cellulose-Mikrofibrillen auf, in welche gleichmäßig Lignin eingelagert ist. Die Markstrahlzellen des Druckholzes erscheinen ebenso stark lignifiziert wie die Markstrahlzellen des Normalholzes. Querschnitte durch diese Zellen lassen die laterale Orientierung des Lignins erkennen. Die Orientierung der Cellulose-Mikrofibrillen in der S ₂-Schicht der zuerst gebildeten Frühholztracheiden des Druckholzes ist dieselbe wie in jenen Zellen, die später ausgeformt werden. Es wird vorgeschlagen, daß zur eindeutigeren Kennzeichnung die äußere ligninreiche Schicht der Druckholztracheiden als S ₂(L)-Schicht bezeichnet wird.

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The authors wish to express their gratitude to Messrs. A. K. Bentum, D. C. Jones, and B. W. Simson for technical assistance. They are also thankful to Dr. D. A. I. Goring, McGill University, Montreal, Canada, for valuable discussions and for making available to them important, unpublished information. This investigation was supported by the United States Department of Agriculture, Forest Service, through Forest Service Research Grant No. 1, which is hereby gratefully acknowledged.     

Comparative study of anatomy and lignin distribution in normal and tension wood of Salix gordejecii

Fibre morphology, anatomy and ultrastructure in cell wall of Salix gordejecii normal wood were examined by transmission electron microscopy (TEM). S. gordejecii tension wood can be recognized anatomically by the presence of gelatinous (G) fibres, which contain a conspicuously thickened inner cell wall layer. TEM images showed that cell wall of S. gordejecii normal wood was typically divided into three layers including the primary wall (P), the middle lamellar (ML) and the secondary wall (S₁, S₂ and S₃). Lignin distribution was determined by using confocal laser scanning microscopy (CLSM) and transmission electron microscopy with energy dispersive X-ray analysis (TEM-EDXA). Confocal images (530 nm) of S. gordejecii normal wood showed strongly lignified CCML, and weakly lignified ML and S2 layer. Weakly lignified fibres (F) and strongly lignified vessels (V) were also detected by using CLSM. Results obtained from confocal microscopy were further confirmed by using TEM-EDXA, indicating that the ratio of lignin concentration in CCML, ML and S2 is 1.72 (1321):1.31 (1006):1 (768). Lignin distribution in tension wood is similar to that in normal wood, except for the non-lignified G layer.     

Mechanical constraints on lignin deposition during lignification

Summary The ultrastructure of lignifying cell walls in Pinus radiata D.Don was investigated using potassium permanganate staining and transmission electron microscopy. Lignin deposition occurred at numerous discrete sites within various cell wall regions, suggesting the presence of some initiating agent at these sites. In the middle lamella region, lignin deposition occurred by addition of protolignin monomers to spherical particles of lignin. Lignification was completed by expansion of these spherical particles, initially forming irregular patterns of lignification followed by infilling of adjacent areas. In contrast, lignification in the secondary wall occurred by deposition of protolignin monomers onto the ends of expanding lignin lamellae between cellulose microfibrils leading to greatly elongated patches of lignin due to the greater rate of deposition along the microfibril axis compared to that across it. It is concluded that the cellulose matrix in which lignin deposition occurs, in the secondary wall, can exert a mechanical influence which limits the rate of lignin deposition in the direction perpendicular to the microfibril axis.     

Studies on opposite wood in conifers part III: Distribution of lignin

Summary The distribution of lignin in opposite wood has been studied by removing the polysaccharides with hydrofluoric acid and examining the resulting lignin skeletons in the electron microscope. The thick S₃ layer was more highly lignified than the S₁ and S₂ layers in Abies balsamea, Picea rubens, Pinus resinosa, and Tsuga canadensis. In Picea rubens, but not in the other species, there was, adjacent to the S₃ layer, a transition zone in S₂ with a high concentration of lignin. The S₃ layer varied considerably in thickness and was often buckled, especially in the latewood. The structure of the bordered pits was that observed in the original wood. The margo, the torus, and the initial pit border were all highly lignified.This paper is dedicated to Dean Edwin C. Jahn in honor of his 70th birthday.     

Helical thickenings and helical cavities in normal and compression woods of Taxus baccata

Summary The longitudinal tracheids in compression wood of Taxus baccata contain helical thickenings but no helical cavities. The thickenings are as frequent and well developed and have the same ropelike appearance as in normal wood of this species. They are an integral part of the S₃ in normal and of the S₂ in compression wood and have the same orientation as the innermost microfibrils in these layers. Except for the absence of cavities and presence of thickenings, compression wood tracheids of Taxus baccata possess all the anatomical features typical of such cells, including a rounded outline, intercellular spaces, a thick S₁ layer, a highly lignified S₂ (L) layer, and no S₃ layer. Pronounced compression wood of Pseudotsuga menziesii contains helical cavities but no helical thickenings. Thickenings and cavities seem to be mutually exclusive in Pseudotsuga and Taxus.This investigation was carried out under the McIntire-Stennis Program, Cooperative State Research Service. I am indebted to Mr. A. Rezanowich of the Pulp and Paper Research Institute of Canada for kindly providing the scanning electron micrographs.     

A contribution to the ultrastructure of tension wood fibers

Summary Pronounced tension wood from four North-American hardwood species has been examined by light and electron microscopy. Delignified fibers were also studied. The gelatinous layer was in all cases loosely attached to S₂ but varied considerably in thickness within each species and was in one case terminated towards the lumen by a layer resembling S₃. A terminal lamella was not observed. After considering both earlier evidence and the present results, it was concluded that the gelatinous layer has neither a honeycomb nor a homogeneous texture, as has been suggested, but that it consists of concentric lamellae of cellulose microfibrils. In the absence of hemicelluloses and lignin, the microfibrils are probably bound together less firmly than they are in other cell wall layers. The gelatinous layer is more readily separated from the remainder of the cell wall by mechanical forces than by chemical reagents.

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Zusammenfassung Zugholz von vier nordamerikanischen Laubbäumen wurde im Licht-und im Elektronenmikroskop ebenso wie entlignifizierte Fasern untersucht. Die gelatinöse Schicht war überall locker an die S₂ gebunden, zeigte aber sehr verschiedene Dicken und war in einem Fall gegen das Lumen von einer Schicht, die der S₃ ähnlich sah, begrenzt. Eine Abschlußlamelle konnte nicht beobachtet werden. Frühere und die eigenen Resultate zeigten, daß die gelatinöse Schicht weder eine Wabenstruktur noch eine homogene Struktur besitzt, sondern daß sie aus konzentrischen Lamellen von Cellulosemikrofibrillen besteht. Da Hemicellulosen und Lignin nicht vorhanden sind, sind die Mikrofibrillen wahrscheinlich nicht so fest aneinander gebunden wie in den anderen Zellwandschichten. Die gelatinöse Schicht läßt sich von der übrigen Zellwand durch mechanische Kräfte leichter als durch chemische Reagenzien trennen.

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This investigation was supported by the United States Department of Agriculture, Forest Service, through Forest Service Research Grant No. 1, which is hereby gratefully acknowledged.     

Microfibril angle of Norway spruce [Picea abies (L.) Karst.] compression wood: comparison of measuring techniques

The structure of cellulose, especially the microfibril angles (MFAs), in compression wood of Norway spruce [Picea abies (L.) Karst.] was studied by wide- and small-angle X-ray scattering and polarizing microscopy. On the basis of the X-ray scattering experiments the average MF As of the cell wall layers S₂ and S₁ of the studied sample are 39 and 89, respectively; and the average diameter and length of the cellulose crystallites are 2.9 and 20.0nm, respectively. The average of the whole MFA distribution is shown to agree with the one obtained by polarizing microscopy of macerated fibers.     

Lignin characterization of triploid clones of Populus tomentosa Carr.

In order to understand the structural characteristics of lignin in triploid clones of Populus tomentosa and its changes in the processes of pulping and bleaching, milled wood lignin (MWL), lignin carbohydrate complex (LCC) and the residual lignin from kraft pulp (KP) and sulfite pulp (SP) were isolated and analyzed by Fourier transform infrared (FTIR) spectrum and ¹³C nuclear magnetic resonance (NMR). The most diagnostic peaks were assigned and the differences were discussed. The spectral patterns reveal that triploid P. tomentosa shows the specific features of hardwood from temperate areas, but in the spectrum of FTIR, the strength ratio of A 1270 cm⁻¹ to A1226 cm⁻¹ is 0.88, higher than the average of hardwood from temperate areas, which will make the lignin delignification more difficult during pulping and bleaching. The LCC from triploid P. tomentosa is mainly composed of xyloglucan and glucuronic acid, and other glucides have much lower ratio. In LCC FTIR, there are three peaks at 1 427, 1 329 and 1 046 cm⁻¹, indicating that both semi-cellulose and cellulose could exist in LCC, and that there might be relationships between cellulose and lignin. Compared with the residual lignin from KP and SP, the condensed structure in KP is more than that in SP.     

A study of lignification in loblolly pine tracheids by the SEM-EDXA technique

Summary The lignification process in different morphological regions of loblolly pine tracheids was studied by the SEM-EDXA technique. Prior to S₂ layer formation, lignification was initiated in the cell corner middle lamella and compound middle lamella regions. Subsequently a rapid lignin deposition was observed in both regions, whereas secondary wall lignification was a more gradual process and initiated when the middle lamella lignin concentration was approximately 50% of maximum. Within the secondary wall, the S₁ layer is lignified first. Then, lagging just behind cell wall formation, lignification of the S₂ layer is initiated adjacent to the S₁ layer and extends toward the lumen. Finally, the S₃ layer lignified. Upon completion of lignification, the cell walls had a higher concentration of lignin in both the S₁ and S₃ layers than in the S₂ layer.This Paper is an excerpt from the Ph.D. dissertation of Shiro Saka     

沙地樟子松天然纯林的结构特征

[目的]为了解红花尔基地区沙地樟子松天然纯林的结构特征,指导沙地樟子松的保护与经营。[方法]在红花尔基地区设置2块100 m×100 m的樟子松天然纯林固定样地,利用样地内每木定位调查数据和分析统计软件进行一元分布及二元分布特征分析。[结果](1)樟子松天然林纯林直径分布为单峰或多峰山状分布,垂直结构简单,只有乔木层和草本层。(2)樟子松天然纯林的林木分布格局为均匀分布,接近随机分布,林木分布格局类型与林分密度无关;林分中樟子松个体竞争激烈,多数单元中林木呈较密集状态。(3)2块样地中随机分布状态下的林木多数为中等密集或比较密集,不同分布状态下的林木优劣性差异较小;低密度樟子松天然纯林中多数密集状态的林木为绝对优势木或优势木,而高密度林分中林木密集度分布与林木大小无关。[结论]红花尔基沙地樟子松天然林结构不合理,应选择病腐木及聚集分布的个体作为潜在调整对象,进行密度调整和结构优化。     

Nanostructural assembly of cellulose,hemicellulose, and lignin in the middle layer of secondary wall of ginkgo tracheid

Physical, chemical, and biological properties of wood depend largely on the properties of cellulose, noncellulosic polysaccharides, and lignin, and their assembly mode in the cell wall. Information on the assembly mode in the main part of the ginkgo tracheid wall (middle layer of secondary wall, S2) was drawn from the combined results obtained by physical and chemical analyses of the mechanically isolated S2 and by observation under scanning electron microscopy. A schematic model was tentatively proposed as a basic assembly mode of cell wall polymers in the softwood tracheid as follows: a bundle of cellulose microfibrils (CMFs) consisting of about 430 cellulose chains is surrounded by bead-like tubular hemicellulose-lignin modules (HLM), which keep the CMF bundles equidistant from each other. The length of one tubular module along the CMF bundle is about 16 ± 2 nm, and the thickness at its side is about 3–4 nm. In S2, hemicelluloses are distributed in a longitudinal direction along the CMF bundle and in tangential and radial directions perpendicular to the CMF bundle so that they are aligned in the lamellae of tangential and radial directions with regard to the cell wall. One HLM contains about 7000 C₆-C₃ units of lignin, and 4000 hexose and 2000 pentose units of hemicellulose.     

Dual-axis electron tomography: a new approach for investigating the spatial organization of wood cellulose microfibrils

We have employed dual-axis electron tomography to investigate the 3D organization of cellulose microfibrils in plastic resin-embedded, delignified cell walls of radiata pine early wood. The ∼ 1 nm thick tomographic slices produced in this study provided for a resolution of ∼ 2 nm in the cross-section of the slices throughout the 150 nm thick plastic sections. This resolution is sufficient to resolve individual cellulose microfibrils and to map the 3D organization of the cellulose microfibrils within the S2 layer of the secondary cell walls. The individual cellulose microfibrils measure ∼ 3.2 nm in diameter, and appear to consist of a ∼ 2.2 nm unstained core and a ∼ 0.5 nm thick surface layer that is lightly stained. Both individual and clustered cellulose microfibrils are seen surrounded by more heavily stained and irregularly shaped residual lignin and hemicellulose. The tightness of packing of the cellulose microfibrils in the cluster varies along the thickness of the section. These findings demonstrate that dual-axis electron tomography is a technique that can provide new insights into the 3D organization of cellulose microfibrils in plant cell walls.     

Micromorphological characteristics of bamboo (<Emphasis Type="Italic">Phyllostachys pubescens</Emphasis>) fibers degraded by a brown rot fungus (<Emphasis Type="Italic">Gloeophyllum trabeum</Emphasis>)

The decay pattern in bamboo fibers caused by a brown rot fungus, Gloeophyllum trabeum, was examined by microscopy. The inner part of the polylaminate secondary wall was degraded, while the outer part of the secondary wall remained essentially intact. Degradation in bamboo fiber walls without direct contact with the fungal hyphae was similar to wood decay caused by brown rot fungi. Degradation in polylaminate walls was almost confined to the broad layers whereas the narrow layers appeared resistant. The p-hydroxylphenyl unit lignin in middle lamella, particularly in the cell corner regions, was also degraded. The degradation of lignin in bamboo fibers was evidenced by Fourier transform infrared spectra. The present work suggests that the decay of bamboo fiber walls by G. trabeum was influenced by lignin distribution in the fiber walls as well as the polylaminate structures.     

黄河小浪底库区山地栓皮栎人工林土壤呼吸的季节动态

[目的]分离并量化土壤自养呼吸和异养呼吸,探讨各自贡献率及其随季节变化的动态特征。[方法]采用壕沟法和气体红外分析法,研究黄河小浪底库区山地栓皮栎人工林土壤总呼吸、自养呼吸和异养呼吸速率的季节动态变化、贡献率和环境影响因子。[结果]表明:栓皮栎人工林总土壤呼吸、自养呼吸和异养呼吸均呈夏季速率高、冬季速率低。栓皮栎土壤总呼吸、自养呼吸及异养呼吸速率与5 cm土壤温度均呈极显著指数相关,温度敏感性系数Q_(10)值大小为自养呼吸(3.40)异养呼吸(2.90)土壤总呼吸(2.45);栓皮栎土壤总呼吸、自养呼吸、异养呼吸速率与0 10 cm土壤体积含水量均显著线性相关;土壤总呼吸、自养呼吸速率与0 10 cm土壤电导率显著相关。土壤总呼吸和异养呼吸的温度敏感系数Q_(10)值均在冬季最大,夏秋季最小;而自养呼吸的Q_(10)值则呈相反的变化趋势。栓皮栎人工林自养呼吸和异养呼吸对土壤总呼吸的月贡献率为13.23%37.33%和62.67%86.76%,且自养呼吸的贡献率与土壤温度的季节变化规律相似。土壤总呼吸、异养呼吸与自养呼吸的CO2年通量分别为1 616.41、1 199.39、417.02 g·m~(-2)·a~(-1)。[结论]经过区分与定量化土壤总呼吸及其组分,确定异养呼吸为本研究区栓皮栎人工林土壤总呼吸的主要组分,作用于异养呼吸的生物与非生物因子均能显著影响整个森林生态系统表层CO_2总排放通量的大小,进一步为该研究区森林生态系统碳循环与能量流动的进一步量化研究提供参考。     

A new norlignan from Taxodium ascendens

A new norlignan, (2R,3R,4S,5S)-2,4-bis(4-hydroxyphenyl)-3,5-dihydroxy-tetrahydropyran (1), together with 9 known compounds were isolated from the branches and leaves of Taxodium ascendens. Their structures were mainly determined on the basis of MS, IR, 1D and 2D NMR spectral evidences. Methanol extract showed inhibitory activity on carbonic anhydrase II with an IC₅₀ value of 4.27 µg/ml, acetone extract and methanol extract inhibited activity of cathepsin B with IC₅₀ values of 2.12 and 3.71 µg/ml, respectively.     

Newly developed nanocomposites from cellulose acetate/layered silicate/poly(ε-caprolactone): Synthesis and morphological characterization

New biodegradable cellulose acetate/layered silicate grafted poly(ε-caprolactone) [(CA/layered silicate)-g-PCL] nanocomposites were prepared by in situ polymerization of ε-caprolactone in the presence of cellulose acetate (CA) and organically modified layered silicate (OMLS). The structures of the resulting composites were investigated. X-ray diffractometry was carried out to survey general structural features of (CA/OMLS)-g-PCL nanocomposites, and revealed that OMLSs having hydroxyl groups in the organic modifiers greatly altered the layered silicate structure by monomer intercalation and successive exfoliation through its polymerization. Two of the representative cases were characterized by wide-angle and small-angle X-ray scattering analyses with a synchrotron source. The morphology of these nanocomposites was further examined by transmission electron microscopy. When SPN, one of OMLSs having one hydroxyl group in its modifier, was used, the silicate layers could not be dispersed thoroughly, but existed as aggregates consisting of several silicate layers. Among them, the crystal growth of PCL developed by transcrystallization, where the crystal growth was restricted in the confined space. When Cloisite 30B, having two hydroxyl groups within the modifier, was used, the silicate layers forming the clay were dispersed completely in the composite and random orientation of the OMLS was observed.     

Molecular cloning of a cellulose synthase gene PtoCesA1 from Populus tomentosa and its genetic transformation in tobacco

A 3 125 bp cellulose synthase gene, PtoCesA1, which has a 98% identity to PtrCesA1 from Populus tremuloides, was cloned from cDNA prepared from secondary xylem of P. tomentosa. Four anti-expression vectors with different fragments of PtoCesA1, named as pBIPF, pBICC1, pBIPR and pBIBR, were constructed. Some traits of transformed tobacco of pBICC1, pBIPR and pBIBR differed from wild types, such as small leaves, “dwarf” phenotype and thinner xylem and fiber cell walls than wild plants consistent with a loss of cellulose. It indicated that the growth of transgenic tobacco was restrained by the expression of anti-PtoCesA1. Transgenic tobacco was obtained and the contents of cellulose and lignin were analyzed as well as the width and length of fiber cells, and xylem thickness for both transgenic and control plants. Transformed tobacco showed a different phenotype from control plants and it implied that PtoCesA1 was essential for the cellulose biosynthesis in poplar stems. [Supported by the Hi-Tech Research and Development Program of China (863) (2001AA244060 and 2003AA244020) and National Basic Research Program of China (973) (J1999016003)]     

Arrangement of cortical microtubules in compression wood tracheids of Taxus cuspidata visualized by confocal laser microscopy

Cortical microtubules (MTs) in differentiating compression wood tracheids of Taxus cuspidata stems were visualized by confocal laser microscopy. They were oriented obliquely at an angle of about 45° to the tracheid axis during formation of the secondary wall. Artificial inclination altered the pattern of alignment of MTs. Banding MTs were helically oriented late during the formation of the secondary walls. These results indicate that MTs might control the orientation and localized deposition of cellulose microfibrils in the secondary walls of compression wood tracheids.Part of this report was presented at the 46th annual meeting of the Japan Wood Research Society, Kumamoto, April 1996     

Soil and microbial respiration in a loblolly pine plantation in response to seven years of irrigation and fertilization

Because soil CO₂ efflux or soil respiration (R_S) is the major component of forest carbon fluxes, the effects of forest management on R_S and microbial biomass carbon (C), microbial respiration (R_H), microbial activity and fine root biomass were studied over two years in a loblolly pine (Pinus taeda L.) plantation located near Aiken, SC. Stands were six-years-old at the beginning of the study and were subjected to irrigation (no irrigation versus irrigation) and fertilization (no fertilization versus fertilization) treatments since planting. Soil respiration ranged from 2 to 6 μmol m⁻² s⁻¹ and was strongly and linearly related to soil temperature. Soil moisture and C inputs to the soil (coarse woody debris and litter mass) which may influence R_H were significantly but only weakly related to R_S. No interaction effects between irrigation and fertilization were observed for R_S and microbial variables. Irrigation increased R_S, fine root mass and microbial biomass C. In contrast, fertilization increased R_H, microbial biomass C and microbial activity but reduced fine root biomass and had no influence on R_S. Predicted annual soil C efflux ranged from 8.8 to 10.7 Mg C ha⁻¹ year⁻¹ and was lower than net primary productivity (NPP) in all stands except the non-fertilized treatment. The influence of forest management on R_S was small or insignificant relative to biomass accumulation suggesting that NPP controls the transition between a carbon source and sink in rapidly growing pine systems.     

Distribution of lignin in normal and tension wood

Summary The distribution of lignin in normal and tension wood of four hardwood species has been studied by examination in the electron microscope of the lignin skeletons remaining after removal of the polysaccharides with hydrofluoric acid. In normal wood fibers, the S₁ had a higher lignin concentration than the S₂ layer, which was not as highly lignified as in conifer tracheids. Vessels had a high concentration of lignin in both normal and tension wood, while the extent of lignification of the parenchyma was variable.In tension wood fibers, the S₁ and S₂ layers were highly lignified. A thick, unlignified G-layer was often associated with an extremely thin S₂ layer with a high concentration of lignin. In both normal and tension wood, the lignin had the same orientation as the cellulose micro-fibrils in the different cell wall layers. The results confirm the earlier conclusion that, in the species investigated, the same amount of lignin is present in gelatinous as in normal fibers. Evidently, the lignification mechanism operates normally in the non-gelatinous layers of the fibers, as well as in the vessels and in the parenchyma of tension wood.

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Zusammenfassung Die Ligninverteilung im Normalholz und im Druckholz von vier Laubhölzern wurde untersucht. Die Ligningerüste, die nach der Entfernung der Polysaccharide durch Fluorwasser-stoffsäure übrigblieben, wurden im Elektronenmikroskop beobachtet. In den Normalholzfasern hatte die S₁-eine höhere Ligninkonzentration als die S₂-Schicht, die weniger lignifiziert war als in den Koniferentracheiden. Die Gefäße hatten eine hohe Ligninkonzentration in sowohl Normal-als in Zugholz, während der Lignifizierungsgrad der Parenchymzellen variierte.In den Zugholzfasern waren die S₁- und S₂-Schichten völlig lignifiziert. Eine dicke, unlignifizierte G-Schicht war oft mit einer außerordentlich dünnen S₂-Schicht, die eine hohe Ligninkonzentration zeigte, verbunden. Sowohl im Normal- wie auch im Zugholz besaß das Lignin dieselbe Orientierung wie die Cellulosemikrofibrillen in den verschiedenen Zellwandschichten. Die Ergebnisse bestätigen den früheren Schluß, daß in den hier untersuchten Laubhölzern in den gelatinösen und in den normalen Fasern dieselbe Ligninmenge vorliegt. Offenbar läuft der Mechanismus der Lignifizierung in den S₁- und S₂-Schichten der gelatinösen Fasern des Zugholzes normal ab.

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This investigation was supported by the United States Department of Agriculture, Forest Service, through Forest Service Research Grant No. 1, which is hereby gratefully acknowledged.