Wood Grain Pattern Formation: A Brief Review

In trees, new wood develops from a layer of stem cells called the vascular cambium. A subpopulation of cambial cells—the fusiform initials—are elongated and capable of coordinated reorientation in response to internal and external stimuli. Changes in the orientation of fusiform initials in turn leads to changes in the grain pattern of developing wood. This article reviews the phenomenon of cambial orientation, with an emphasis on a recent computer model that takes the plant hormone auxin as the orienting signal. New model results are presented that demonstrate the surprisingly complex grain patterns that can emerge from simple initial conditions, in qualitative agreement with similar patterns found in wood. Lastly, an alternative theory of wood grain pattern that takes mechanical stress as the orienting signal is critically evaluated.     

VASCULAR CAMBIUM AND WOOD DEVELOPMENT IN CARBONIFEROUS PLANTS. I. LEPIDODENDRALES

Patterns of activity in the vascular cambium of Carboniferous arborescent lycopods (Paralycopodites and Stigmaria) were studied by analysis of serial tangential sections of the secondary xylem. The analysis assumes that cell patterns in the wood accurately reflect those of the corresponding cambium. An evaluation using indirect evidence indicates that the assumption is valid as far as can be determined from comparison with living plants. The tracheids of the secondary xylem enlarge in a centrifugal pattern, suggesting a progressive enlargement of the fusiform initials. There is no evidence of periodic anticlinal division of these initials, and it is proposed that the increase in cambial circumference was accommodated primarily by an increase in fusiform initial size. In some axes with abundant secondary xylem there is evidence that isolated initials or groups of initials sporadically subdivided to form numerous, spindle-shaped meristematic cells. Some of these cells subsequently developed into typical cambial initials. Tissues presumably formed during the cessation of cambial growth in Lepidodendron and Stigmaria are described; the structure of the tissues is suggestive of a postmeristematic parenchymatous sheath. It is concluded that cambial activity in these arborescent cryptogams was clearly different from that of modern seed plants, further attesting to the distinctive nature of this ancient group.     

MIGRATING CAMBIAL DOMAINS AND THE ORIGIN OF WAVY GRAIN IN XYLEM OF BROADLEAVED TREES

Anatomical analysis of wavy-grained xylem samples of Fraxinus, Acer, and Betula reveals a strong underlying pattern of orientational domains within the cambium. Domains are local areas in the cambium within which the orientation of pseudotransverse divisions of fusiform initial cells is predominately unidirectional rather than random. The orientation of ray splitting and ray uniting also tends to be unidirectional within domains. Because their boundaries migrate with time, domains are not fixed in shape or position. A specific group of initial cells may first be located in the interior of a domain, later near a boundary, and still later in another domain of opposite orientational prevalence. Domain patterns and their movements were delineated by classifying and mapping ray splitting and uniting events in temporal series of tangential sections of terminal late xylem. The domain patterns move slowly upward, preceding the similarly outlined, but noncoincident, grain pattern. A wavy grain pattern can be interpreted as being the result of a domain pattern of similar geometry. The efficacy of a domain pattern in eliciting a grain pattern is probably indirectly dependent upon the frequency of occurrence of pseudotransverse divisions and of ray splitting and uniting. The frequency is low in cambium producing straight-grained wood. Evidence of 10-fold and greater increases in frequency of ray splitting and uniting was found in straight-grained to wavy-grained transitional xylem.     

喜树原形成层到形成层转化的研究

观察了枣树茎中原形成层到形成层的转化过程。距茎端0．5mm处,节间开始伸长之前,横切面上4—5个原形成束及束间的分生组织组成原形成层环。径向切面观,原形成层环呈现出较均一的结构。随着节间开始伸长,由于原形成层细胞发生假横向分裂,出现了长短两类细胞,长细胞多数端壁倾斜,短细胞多数端壁平截。以后,长细胞发育为纺缍状原始细胞,短细胞发育为射线原始细胞,部分射线原始细胞可以伸长井侵入生长而转化为纺缍状原始细胞。在节间伸长将停止时,此种转化基本完成。喜树为非叠生形成层,纺缍状原始细胞和射线原始细胞都有多核现象发生。     

The origin and development of vascular cambium in girdled stems ofEucommia ulmoides Oliv.

We conducted anatomical studies of girdled stems ofEucommia ulmoides at various developmental stages to elucidate the origin and development of callus and the vascular cambium. In the transverse view, ray initial cells in the cambial zone began to divide both periclinally and anticlinally 2 d after girdling. Fusiform initial cells started to enlarge at 3 d, then gradually proliferated via periclinal divisions. Thus, the callus was derived from the ray initial cells of the cambial zone as well as from fusiform initial cells. In the tangential view, callus cells derived from ray initial cells were short while those from fusiform initial cells were long, thereby producing a heterogeneous structure. However, the fusiform initial cells underwent transverse divisions 10 d after girdling, which resulted in shorter cells and a homogeneous callus structure. Afterward, some short cells divided transversely while others elongated, so that a heterogeneous form was regained. Finally, the vascular meristem that was girdled early in its development redifferentiated from short and long cells in the callus. The long cells developed into fusiform initials, with the short ones becoming ray initials.     

Mathematical Modeling of Intrusive Growth of Fusiform Initials in Relation to Radial Growth and Expanding Cambial Circumference in Pinus sylvestris L.

This study on the cambium of Pinus sylvestris L. examines the intrusive growth of fusiform cambial initials and its possible contribution to the tangential and radial expansions of the cambial cylinder. The location and extent of intrusive growth of the fusiform initials were determined by microscopic observations and by mathematical modeling. In order to meet the required circumferential expansion of the cambial cylinder, the fusiform initials grow in groups by means of a symplastic rather than intrusive growth, leaving no room for the assumption that intrusive growth of the initials takes place between radial walls and has a direct role in the increase of the cambial circumference. Therefore, it is postulated that the fusiform initials grow intrusively between the tangential walls of the neighboring initials and their immediate derivatives and not between the radial walls of the adjacent initials as per common belief.     

THE VASCULAR CAMBIUM AND XYLEM DEVELOPMENT IN HIBISCUS LASIOCARPUS

Cumbie, B. G. (U. Missouri, Columbia.) The vascular cambium and xylem development in Hibiscus lasiocarpus. Amer. Jour. Bot. 50(9): 944–951. Illus. 1963.—Circumferential growth of the vascular cambium, as determined primarily by an analysis of the secondary xylem, in Hibiscus lasiocarpus, an herbaceous dicotyledon, occurred through both radial and oblique anticlinal divisions. Divisions to produce segments were less frequent. Although the fusiform initials usually elongated somewhat between successive divisions, this accounted for very little increase in circumference of the cambium. A fusiform initial underwent a specific pattern of anticlinal divisions, determined primarily by its length, at the beginning of cambial activity. There was no loss of fusiform initials, except by ray formation. Most new rays originated only after considerable secondary xylem had been formed. The findings are discussed in relation to circumferential growth of the vascular cambium in woody dicotyledons.     

DEVELOPMENTAL CHANGES IN THE VASCULAR CAMBIUM IN LEITNERIA FLORIDANA

Developmental changes in the vascular cambium of Leitneria floridana, a shrub, were determined primarily by an analysis of the secondary xylem. During the production of the first growth ring of secondary xylem, 37% of the anticlinal divisions in the fusiform initials were lateral, the remaining were oblique. The oblique partition averaged ½ of the length of the dividing initials during this period of growth. Following their origin in anticlinal division, daughter cells elongated at a rapid rate until they were about as long as the mean for all cells, and then most cells elongated at a slow rate. Almost all initials survived during the formation of the inner secondary xylem (growth rings 1–10), and few new rays were formed from fusiform initials. During the production of the outer secondary xylem (growth rings 22–26), lateral divisions accounted for less than 5% of all anticlinal divisions. The oblique partition averaged only ¼ of the length of the dividing cells during this period, although the mean length of dividing initials was relatively constant throughout secondary growth. About 20% of the initials studied during the deposition of the outer secondary xylem disappeared from the cambium, and many others were transformed into ray initials. The findings are discussed in relation to the developmental changes in the vascular cambium in plants of different habits.     

Rearrangement of cells in storeyed cambium of<Emphasis Type="Italic"> Lonchocarpus sericeus</Emphasis> (Poir.) DC connected with formation of interlocked grain in the xylem

The history of cellular events in the storeyed cambium of Lonchocarpus sericeus (Poir.) DC was analysed on the basis of changes in the cell arrangement in successive layers and strata of axial parenchyma in the xylem. The mechanism of formation of the regular interlocked grain was investigated. Inclination of fusiform cells changes intensively whereas height and position of storeys in the successive layers of axial parenchyma are constant. As a result, new contacts between cells are formed by means of the intrusive growth of ends of cells belonging to one storey between the tangential walls of cells of the neighbouring storey and unequal periclinal divisions, which give a new shape to the initials. The concept of intrusive growth between the radial walls of the fusiform initials in the formation of xylem with interlocked grain should be revised on this basis.     

Variations in the Lengths of Fusiform Cambial Cells and Vessel Elements in Kalopanax pictus

Samples of a mature specimen of Kalopanax pictus, a ring-poroushardwood, were studied to compare the respective lengths offusiform cambial cells and vessel elements in the stem. Thelengths of dormant and reactivated fusiform cambial cells weremeasured with a confocal laser scanning microscope in tissuethat had been macerated by digestion with pectinase and in thicktangential sections. The lengths of early wood and late woodvessel elements were measured in tissues that had been maceratedby Franklin's method. The vessel elements and fusiform cambialcells varied considerably in length within individual samples.The mean length of early wood vessel elements corresponded tothat of fusiform cells in the dormant cambium but not in thereactivated cambium. Significant differences were observed betweenthe mean lengths of dormant and reactivated fusiform cambialcells, between those of reactivated fusiform cambial cells andearly wood vessel elements, between those of reactivated fusiformcambial cells and late wood vessel elements, and between thoseof early wood and late wood vessel elements. The frequency distributionsof lengths of cambial cells were bimodal and differed from thoseof vessel elements, which more closely resembled a normal distribution.The proportion of shorter lengths was higher in the reactivatedcambium than in the dormant cambium, the early wood and thelate wood vessel elements. Our results do not suppot the hypothesisthat the lengths of early wood vessel elements in ring-poroushardwoods change during differentiation. The similar rangesof recorded lengths suggest that short and long vessel elementsmight be derived directly from short and long cambial cells,respectively. Copyright 1999 Annals of Botany Company Kalopanax pictus, cambium, vessel element, confocal laser scanning microscopy, maceration, cell length.     

INTERLOCKED GRAIN,CAMBIAL DOMAINS,ENDOGENOUS RHYTHMS,AND TIME RELATIONS,WITH EMPHASIS ON NYSSA SYLVATICA

Interlocked grain in wood of Nyssa sylvatica Marsh. is a lapse-time record of cyclical changes in inclination (rightward or leftward) of fusiform initial cells in the cambium. Typically the inclination cycles have periods of a decade or more. Such cycles can be interpreted as manifestations of long-term endogenous rhythms. Interlocked grain in N. sylvatica is not the result of whole-stem spiral grain that reverses periodically. As wavy grain in other species is a record of wavelike migration of orientational domains along the cambium, interlocked grain in N. sylvatica can be interpreted as being related to a more complex system of domains having long axial dimensions. These domains migrate or change their efficacy and directional sense in place in such a way that some regularity of inclination cycles is maintained at the various stem levels. Inclination cycle lengths have a closer relation to calendar time than they do to the amount of radial growth of the stem. This is affirmed by data from paired stems of similar ages but which grew in diameter at widely different rates.     

EFFECTS OF BALSAM WOOLLY APHID (ADELGES PICEAE) INFESTATION ON CAMBIAL ACTIVITY IN ABIES GRANDIS

Salivary secretions injected into the cortex or outer phloem by Adelges piceae (balsam woolly aphid) feeding on Abies grandis induce the production of wood that is in some respects similar to compression wood. Cambial activity was analyzed by examination of serial tangential sections through an annual ring in the xylem produced before infestation occurred and compared with similar sections from a ring produced after infestation. Growth after infestation was characterized by increased periclinal and anticlinal divisions of fusiform initials, increased production of new ray initials from fusiform initials and from anticlinal divisions of existing ray initials, and decline of numerous fusiform initials and termination of many tiers by maturation. This results in increased frequency of ray fusion and separation by decline or intrusion of adjacent fusiform initials. There was a marked increase in size and number of rays and number of parenchyma strands both of which also distinguish aphid-affected wood from compression wood with which it frequently has been compared.     

Modelling for rearrangement of fusiform initials during radial growth of the vascular cambium in Pinus sylvestris L.

In contrast to common belief, recent studies have confirmed that intrusive growth of fusiform cambial initials has a significant role in the rearrangement of the initials, but does not contribute to the cambial circumference increment. We observed a rapid rearrangement of cambial initials on a long series of transverse sections of the vascular cambium and the wood of a 50-year-old pine (Pinus sylvestris L.) tree. A comparison of cell arrangement in consecutive sections, as well as a critical analysis of tangential reconstructions, has confirmed that changes in cell locations in a group of cells on the tangential surface caused no change in the total tangential width of the whole group. Models illustrating changes in locations of the initials have been proposed, assuming that intrusive growth, which makes the growing initials intrude between the neighbouring initials and their immediate derivatives, is localized on the longitudinal edges of cells. We infer that intrusive growth of the cambial initials in P. sylvestris is not involved in the cambial circumference increment, but plays a significant role in the rearrangement of the initials, probably allowing for a relaxation of shearing strains generated during radial growth. The relationship of intrusive growth with the elimination of initials has been discussed with reference to the frequency of anticlinal divisions. It has been proposed that the occurrence of anticlinal divisions in excess over the actual requirement for increase in the cambial circumference could be due to internal shearing strains.     

Environmental and auxin regulation of wood formation involves members of the Aux/IAA gene family in hybrid aspen

Indole acetic acid (IAA/auxin) profoundly affects wood formation but the molecular mechanism of auxin action in this process remains poorly understood. We have cloned cDNAs for eight members of the Aux/IAA gene family from hybrid aspen (Populus tremula L. x Populus tremuloides Michx.) that encode potential mediators of the auxin signal transduction pathway. These genes designated as PttIAA1-PttIAA8 are auxin inducible but differ in their requirement of de novo protein synthesis for auxin induction. The auxin induction of the PttIAA genes is also developmentally controlled as evidenced by the loss of their auxin inducibility during leaf maturation. The PttIAA genes are differentially expressed in the cell types of a developmental gradient comprising the wood-forming tissues. Interestingly, the expression of the PttIAA genes is downregulated during transition of the active cambium into dormancy, a process in which meristematic cells of the cambium lose their sensitivity to auxin. Auxin-regulated developmental reprogramming of wood formation during the induction of tension wood is accompanied by changes in the expression of PttIAA genes. The distinct tissue-specific expression patterns of the auxin inducible PttIAA genes in the cambial region together with the change in expression during dormancy transition and tension wood formation suggest a role for these genes in mediating cambial responses to auxin and xylem development.     

Organisation of vascular cambium during different seasons in some tropical timber trees

The vascular cambium of Albizzia, Tectona, Terminalia, Calophyllum, Mangifera and Morinda was non-storied while that of Dalbergia was semi-storied. The ray initials were uniseriate in Terminalia and Calophyllum , both uniseriate and biseriate in Albizzia, Dalbergia, Mangifera and Morinda and were also multiseriate in Tectona . They were homogeneous in Albizzia and heterogeneous in the other species. The radial walls of cambial cells were always beaded, although beads were more prominent and closer to one another during periods of cambial dormancy than during activity. The fusiform initials were comparatively less vacuolated during dormancy/ least activity. When active, the fusiform and ray initials of all species, except Calophyllum , also showed multinucleate condition (2–10 depending on species). The proportion of ray initials to fusiform initials was almost constant throughout the year in all species.     

Ontogeny of the interfascicular cambium in the petiole ofTabebuia rosea DC. (Bignoniaceae)

In order to elucidate the origin of interfascicular cambium in the petiole ofTabebuia rosea, transverse and tangential views of the cells in the interfascicular region during the different developmental stages of the petiole have been traced. Interfascicular cambium originates from the interfascicular parenchyma, which has been differentiated from interfascicular vascular meristem cells. Interfascicular parenchyma cells divide periclinally differentiating into the interfascicular metacambial cells and then into the cambium. Tangentially the homogenous structure of interfascicular parenchyma cells in the early stage gradually changes into a heterogenous structure with long and short cells from which fusiform and ray initials are derived, respectively. Ontogenetic pattern of the interfascicular cambium is similar to that of the fascicular one with interfascicular metacambium as an intermediate stage.     

Developmental anatomy of vascular cambium and periderm ofBotrypus virginianus and its bearing on the systematic position of ophioglossaceae

The developmental anatomy of the vascular cambium and periderm ofBotrypus virginianus was studied, and its bearing on the systematic position of Ophioglossacease is discussed. The cambial zone including cambium is initiated in a procambial ring of the stem before primary vascular tissue is well differentiated. The presumed cambium is composed of fusiform and ray initials. The cambium is extremely unequally bifacial, producing secondary xylem centripetally, and quite a small number of parenchymatous cells but no secondary phloem centrifugally. The cambial activity persists long, although it is very low in the mature part of the stem. It seems that the circumferential increase of the cambium is accommodated by an increase in the number of cambial initials. Secondary xylem is nonstoried and composed of tracheids with circular-bordered pits with evenly thick pit membranes, and uniseriate or partly biseriate radial rays. It makes up the bulk of the stem xylem. Periderm is formed almost entirely around the stem, simultaneous with its increment due to the secondary xylem. The combination of these anatomical features of secondary tissue supports the idea that Ophioglossaceae are living progymnosperms.     

A secondary phloic (bast) fibre-shy (<Emphasis Type="Italic">bfs</Emphasis>) mutant of dark jute (<Emphasis Type="Italic">Corchorus olitorius</Emphasis> L.) develops lignified fibre cells but is defective in cambial activity

Bast fibre development in jute (Corchorus spp.) is a complex process that involves the differentiation of secondary phloic fibres (SPF) from the cambium followed by lignification of the fibre wall. We have identified a unique radiation-induced bast fibre-shy mutant of dark jute (C. olitorius L.), which is concurrently defective in the differentiation of SPF and secondary xylem (wood) but develops lignified fibre cells. It displays the most unusual phenotype with stunted growth and abnormal leaf shape, matures earlier, yields significantly less bast fibres and wood, and produces poorer quality fibres than its parental wild-type. Cambial activities in the mutant and the normal type were monitored by estimating the fibre content that entails the total number of fibre cell bundles (FCBs) in an entire transversal section. The results show that a multi-fold reduction of bast fibre yield in the mutant is related to development-specific loss of cambium function along the length of the stem from to top to bottom. Since lignification of the fibre wall in the mutant is not only normal but also developmentally uniform, cambium function may be unrelated to the lignification process during bast fibre development. Lignin does not influence bast fibre strength and fineness. The architecture of the mostly triangular FCB wedges, which is governed by a balanced growth between radially elongating FCBs and tangentially expanding ray cells due to development-specific activation of the fusiform and ray initials of the cambium, conditions fibre fineness. Our study shows that mutation could specifically impair the cambial activity by rendering those initials that differentiate the SPF and secondary xylem nonfunctional.     

ANATOMY OF NODES VS. INTERNODES IN COLEUS: THE NODAL CAMBIUM

Secondary growth begins in the nodal regions before the internodal regions in Coleus, so that longitudinally discontinuous vascular cambia are formed in the 6th through the 9th or 10th nodes, where the internodal cambium becomes continuous between nodal cambia. The nodal cambia are identifiable by radial seriation in interfascicular regions, typical cytology of fusiform initials, and the presence of a ray system. Anatomical features distinct from the primary plant body are shared by the nodal and internodal cambia. Branching of primary vascular strands, restricted to procambium and phloem, is virtually confined to nodal regions. In secondary growth, vascular branching of xylem and phloem occurs in both nodes and internodes. Xylem strand branches are formed only from derivatives of vascular cambia. It is proposed that the cambium provides the secondary plant body an efficient channel for lateral auxin transport, by which branching across interfascicular regions is facilitated.