Relationships between cambial activity,cell differentiation and the localization of starch in storage tissues around the cambium in locally heated stems of<Emphasis Type="Italic"> Abies sachalinensis</Emphasis> (Schmidt) Masters

A study was made, in a cool-temperate zone, of the extent of cell division in the cambium, the extent of differentiation of cambial derivatives, and the localization of storage starch around the cambium in locally heated (22–26°C) stems of the evergreen conifer Abies sachalinensis (Schmidt) Masters during cambial dormancy and immediately after natural reactivation of the cambium. In locally heated regions of stems during cambial dormancy, heating induced localized reactivation of the cambium. However, the cells in the heated and reactivated cambium stopped dividing soon after only a few cells had been generated. In addition, no differentiation of the xylem and the disappearance of starch from storage tissues around the cambium were observed. In regions of stem that had been locally heated after natural reactivation of the cambium, cell division continued in the cambium and earlywood tracheids with a large radial diameter and secondary walls were formed, with abundant starch in the storage tissues around the cambium. Our results suggest that the extent of both cell division in the cambium and cell differentiation depends on the amount of starch in storage tissues around the cambium in the locally heated stems of an evergreen conifer growing in a cool-temperate zone.     

Induction of cambial reactivation by localized heating in a deciduous hardwood hybrid poplar (Populus sieboldii x P. grandidentata)

BACKGROUND AND AIMS: The timing of cambial reactivation plays an important role in the control of both the quantity and the quality of wood. The effect of localized heating on cambial reactivation in the main stem of a deciduous hardwood hybrid poplar (Populus sieboldii x P. grandidentata) was investigated. METHODS: Electric heating tape (20-22 degrees C) was wrapped at one side of the main stem of cloned hybrid poplar trees at breast height in winter. Small blocks were collected from both heated and non-heated control portions of the stem for sequential observations of cambial activity and for studies of the localization of storage starch around the cambium from dormancy to reactivation by light microscopy. KEY RESULTS: Cell division in phloem began earlier than cambial reactivation in locally heated portions of stems. Moreover, the cambial reactivation induced by localized heating occurred earlier than natural cambial reactivation. In heated stems, well-developed secondary xylem was produced that had almost the same structure as the natural xylem. When cambial reactivation was induced by heating, the buds of trees had not yet burst, indicating that there was no close temporal relationship between bud burst and cambial reactivation. In heated stems, the amount of storage starch decreased near the cambium upon reactivation of the cambium. After cambial reactivation, storage starch disappeared completely. Storage starch appeared again, near the cambium, during xylem differentiation in heated stems. CONCLUSIONS: The results suggest that, in deciduous diffuse-porous hardwood poplar growing in a temperate zone, the temperature in the stem is a limiting factor for reactivation of phloem and cambium. An increase in temperature might induce the conversion of storage starch to sucrose for the activation of cambial cell division and secondary xylem. Localized heating in poplar stems provides a useful experimental system for studies of cambial biology.     

Changes in the localization and levels of starch and lipids in cambium and phloem during cambial reactivation by artificial heating of main stems of Cryptomeria japonica trees

Background and Aims

Cambial reactivation in trees occurs from late winter to early spring when photosynthesis is minimal or almost non-existent. Reserve materials might be important for wood formation in trees. The localization and approximate levels of starch and lipids (as droplets) and number of starch granules in cambium and phloem were examined from cambial dormancy to the start of xylem differentiation in locally heated stems of Cryptomeria japonica trees in winter.

Methods

Electric heating tape was wrapped on one side of the stem of Cryptomeria japonica trees at breast height in winter. The localization and approximate levels of starch and lipids (as droplets) and number of starch granules were determined by image analysis of optical digital images obtained by confocal laser scanning microscopy.

Key Results

Localized heating induced earlier cambial reactivation and xylem differentiation in stems of Cryptomeria japonica, as compared with non-heated stems. There were clear changes in the respective localizations and levels of starch and lipids (as droplets) determined in terms of relative areas on images, from cambial dormancy to the start of xylem differentiation in heated stems. In heated stems, the levels and number of starch granules fell from cambial reactivation to the start of xylem differentiation. There was a significant decrease in the relative area occupied by lipid droplets in the cambium from cambial reactivation to the start of xylem differentiation in heated stems.

Conclusions

The results showed clearly that the levels and number of storage starch granules in cambium and phloem cells and levels of lipids (as droplets) in the cambium decreased from cambial reactivation to the start of xylem differentiation in heated stems during the winter. The observations suggest that starch and lipid droplets might be needed as sources of energy for the initiation of cambial cell division and the differentiation of xylem in Cryptomeria japonica.     

Relationship between the earlywood-to-latewood transition and changes in levels of stored starch around the cambium in locally heated stems of the evergreen conifer <Emphasis Type="Italic">Chamaecyparis pisifera</Emphasis>

Key message

We observed the formation of latewood tracheids with narrow diameters and thick walls and the disappearance of stored starch around the cambium on the locally heated region of stems in evergreen conifer Chamaecyparis pisifera during winter cambial dormancy.

Abstract

Wood formation is controlled by cambial cell division, which determines the quantity and quality of wood. We investigated the factors that control cambial activity and the formation of new tracheids in locally heated stems of the evergreen conifer Chamaecyparis pisifera. Electric heating tape was wrapped around one side of the stem, at breast height, of two trees in 2013 and two in 2014. Pairs of stems were locally heated in winter, and small blocks were collected from heated and non-heated regions of stems. Cambial activity and levels of stored starch around the cambium were investigated by microscopy. Cambial reactivation and xylem differentiation occurred earlier in heated than in non-heated regions. New cell plates were formed after 14–18 days of heating. After a few layers of tracheids with large diameters and thin walls had formed, cell division and cell enlargement during differentiation were inhibited. Tracheids with narrow diameters and thick walls, defining those as latewood, were formed near the cambium, and finally, four to six layers of tracheids were induced. After cambial reactivation, amounts of stored starch started to decrease and starch disappeared completely from phloem and xylem cells that were located near the cambium during the differentiation of heated regions. Our results suggest that an increase in temperature induces the conversion of stored starch to soluble sugars for continuous cambial cell division and earlywood formation. By contrast, a shortage of stored starch might be responsible for inhibition of cambial activity and induction of the formation of latewood tracheids.

     

Immunolocalization of cinnamyl alcohol dehydrogenase 2 (CAD 2) indicates a good correlation with cell-specific activity of CAD 2 promoter in transgenic poplar shoots

Cinnamyl alcohol dehydrogenase 2 (CAD 2) localization and the cell-specific activity of the eucalyptus CAD 2 promoter were investigated by CAD 2 immunogold localization and promoter β-glucuronidase (GUS) histochemistry in apical and mature parts of stable transformed poplar (Populus tremula × P. alba) stems. Both CAD 2 protein and GUS activity were found to be confined in the same types of cells in the shoot apices, particularly in the determined meristematic cells in leaf axils and shell zones, procambium and developing tracheids. Within mature stems, CAD 2 and GUS were also identified in cambium and in fully or partially lignified cells derived from it (young xylem, developing phloem fibres, chambered parenchyma cells around phloem). Additionally, GUS activity was found in the scale leaves of apical shoot buds and in the roots (namely in the procambium, cambium, phellogen, young xylem, pericycle) of transformed plants. By employing immunogold cytochemistry, CAD 2 was shown to be localized in the cytoplasm within cambial, ray and young xylem cells in stems, the gold particles being randomly attached to endoplasmic reticulum and Golgi-derived vesicles. These results support a crucial role for CAD 2 in lignification and indicate a new role for this enzyme in branching events within the shoot apex and during lateral root formation. Received: 24 April 1997 / Accepted: 17 July 1997     

Immunolocalization indicates plasmodesmal trafficking of storage proteins during cambial reactivation in Populus nigra

Background and Aims

Cambium reactivation after dormancy and budbreak in deciduous trees requires a supply of mobilized reserve materials. The pathway and mode of transfer of these materials are poorly understood.

Methods

Transport of reserve materials during cambium reactivation in Populus nigra was investigated by conventional and immunocytochemical TEM analyses, SDS–PAGE, western blotting and intracellular microinjection of fluorescent dyes.

Key Results

Proteinaceous compounds stored in vacuoles and protein bodies of vascular cells and ray cells disappeared within 3 weeks after cambial reactivation and budbreak. Some of these proteins (32 kDa, 30 kDa and 15 kDa) were labelled by lectin antibodies in SDS–PAGE. The same antibodies were localized to plasmodesmata (PDs) between phloem parenchyma, ray cells and fusiform cambial cells. In addition, proteinaceous particles were localized inside the cytoplasmic sleeves of these PDs during budbreak. During this period, the functional diameter of PDs was about 2·2 nm which corresponds approximately to the Stokes'' radius of the detected 15-kDa protein.

Conclusions

Lectin-like reserve proteins or their degradation products seem to be transferred through PDs of phloem parenchyma and rays during cambial reactivation and budbreak. PD transfer of storage proteins is a novelty which supports the concept of symplasmic nutrient supply to the cambial region.     

Cambial sensitivity to rising temperatures by natural condition and artificial heating from late winter to early spring in the evergreen conifer Cryptomeria japonica

Differences in the timing of cambial reactivation and the initiation of xylem differentiation in response to the sum of daily maximum temperatures were studied in two Cryptomeria japonica trees with cambium of different ages under natural and locally heated conditions. In addition, we observed the effects of low temperature on cambial activity. The timing of cambial reactivation and of the initiation of xylem differentiation differed between 55- and 80-year-old cambium under natural conditions. In the 55-year-old cambium, cambial reactivation occurred when the cambial reactivation index (CRI), calculated on the basis of daily maximum temperatures in excess of 10°C, was 94 and 97°C in 2007 and 2008, respectively. In 80-year-old cambium, cambial reactivation occurred when the CRI, calculated on the basis of daily maximum temperatures in excess of 11°C, was 69 and 71°C in 2007 and 2008, respectively. After cambial reactivation in 2007, normal cell division was evident in the cambium even though the minimum temperature had fallen between −2 and −3°C. Under natural conditions, xylem differentiation started 38–44 days after cambial reactivation. In heated stems, the time between cambial reactivation and the initiation of xylem differentiation ranged from 14 to 16 days, a much shorter time than under natural conditions, indicating that continuous exposure to an elevated temperature had induced earlier xylem differentiation. Our observations indicate that the sensitivity to reactivation inducing stimuli of the cambium depends on both the stage of dormancy and tree age of the cambium.     

Cold stability of microtubules in wood-forming tissues of conifers during seasons of active and dormant cambium

The cold stability of microtubules during seasons of active and dormant cambium was analyzed in the conifers Abies firma, Abies sachalinensis and Larix leptolepis by immunofluorescence microscopy. Samples were fixed at room temperature and at a low temperature of 2–3°C to examine the effects of low temperature on the stability of microtubules. Microtubules were visible in cambium, xylem cells and phloem cells after fixation at room temperature during seasons of active and dormant cambium. By contrast, fixation at low temperature depolymerized microtubules in cambial cells, differentiating tracheids, differentiating xylem ray parenchyma and phloem ray parenchyma cells during the active season. However, similar fixation did not depolymerize microtubules during cambial dormancy in winter. Our results indicate that the stability of microtubules in cambial cells and cambial derivatives at low temperature differs between seasons of active and dormant cambium. Moreover, the change in the stability of microtubules that we observed at low temperature might be closely related to seasonal changes in the cold tolerance of conifers. In addition, low-temperature fixation depolymerized microtubules in cambial cells and differentiating cells that had thin primary cell walls, while such low-temperature fixation did not depolymerize microtubules in differentiating secondary xylem ray parenchyma cells and tracheids that had thick secondary cell walls. The stability of microtubules at low temperature appears to depend on the structure of the cell wall, namely, primary or secondary. Therefore, we propose that the secondary cell wall might be responsible for the cold stability of microtubules in differentiating secondary xylem cells of conifers.     

Seasonal variation in the ultrastructure of the cambium in young stems of willow (Salix viminalis) in relation to phenology

The growth period of Salix viminalis L. (clone 683) plants near Stockholm, Sweden, (59.5°N, 18.3°E) started in April with flowering and ended in October with abscission of the shoot tips. Cell divisions in the vascular cambium started almost two months before sprouting and ceased at about the same time as the elongation growth of the shoots. Phloem cells were apparently produced before flowering, while new xylem production started at the time of flushing. Cytodifferentiation in immature xylem continued until November. Thick-walled cells with protoplasm were observed adjacent to xylem mother cells in the cambium during the winter. The number of radially arranged cells in the cambial zone increased from 3–4 during dormancy to about 18 during the mitotic maximum in July. Seasonal variation was apparent in vacuolization, wall thickness and presence of storage material in the cells. Lipid bodies and protein bodies occurred in both fusiform and ray initials, while starch was observed in ray initials, ray cells and in the phloem. In September the ultrastructure of the cambium showed anatomical features characteristic for both active and dormant cells. Dictyosomes with vesicles and rough ER were present in thick-walled cells that contained lipid bodies and starch granules. Nuclear divisions in the cambium ended in October.     

Influence of exogenous ethylene on cambial activity,xylogenesis and ray initiation in young shoots of Leucaena leucocephala (lam.) de Wit

The effect of exogenous ethephon on cambial activity, xylem production and ray population in young shoots of Leucaena leucocephala was investigated anatomically. The application of ethephon showed a diphasic effect on cambial activity and xylogenesis in a dose dependent manner. Lower concentration of ethephon enhanced cambial activity while high concentrations reduced cambial cell divisions and daughter-cell differentiation. High ethephon concentration also resulted in shorter vessel elements, thick walled fibers and phenolic accumulation in ray cells and vessel elements, whereas low concentration allowed elongation of fibers and vessel elements. The density of rays increased significantly with increase in ethylene concentration. The evaluation of longitudinal sections of cambial zone in ethephon treated plants showed high frequency of transformation of fusiform initials into ray initials through divisions and segmentation, resulting in high ray frequency in both xylem and phloem. The present study demonstrates that ethylene plays an important role in regulating secondary vascular tissue composition by reducing the population of fusiform initials in the cambium.     

Ultrastructure of cell division in the fusiform cells of the vascular cambium of Robinia pseudoacacia

 The ultrastructure of periclinally dividing fusiform cells was studied in the vascular cambium of Robinia pseudoacacia. Fusiform cell division begins in April at Madison, Wisconsin, when the cambial cells still have many characteristics of a dormant cambium. Soon afterward, the cambial cells acquire the appearance typical of an active cambium. Sequential phases of the microtubule cycle were documented: cortical microtubules bordering the cell wall during interphase, perinuclear microtubules preceding formation of the mitotic spindle, spindle microtubules, and phragmoplast microtubules. A preprophase band of microtubules was not encountered. An extended phragmosome was not encountered in periclinally dividing fusiform cells. During cytokinesis, the phragmosome is represented by a broad cytoplasmic plate which precedes the developing phragmoplast and cell plate as they migrate toward the ends of the cell.     

Stem anatomy of Dolichos lablab Linn (Fabaceae): Origin of cambium and reverse orientation of vascular bundles

Secondary growth in the stem of Dolichos lablab is achieved by the formation of eccentric successive rings of vascular bundles. The stem is composed of parenchymatous ground tissue and xylem and phloem confined to portions of small cambial segments. However, development of new cambial segments can be observed from the obliterating ray parenchyma, the outermost phloem parenchyma and the secondary cortical parenchyma. Initially cambium develops as small segments, which latter become joined to form a complete cylinder of vascular cambium. Each cambial ring is functionally divided into two distinct regions. The one segment of cambium produces thick-walled lignified xylem derivatives in centripetal direction and phloem elements centrifugally. The other segment produces only thin-walled parenchyma on both xylem and phloem side. In mature stems, some of the axial parenchyma embedded deep inside the xylem acquires meristematic activity and leads to the formation of thick-walled xylem derivatives centrifugally and phloem elements centripetally. The secondary xylem comprises vessel elements, tracheids, fibres and axial parenchyma. Rays are uni-multiseriate in the region of cambium that produces xylem and phloem derivatives, while in some of the regions of cambium large multiseriate, compound, aggregate and polycentric rays can be noticed.     

Cambial activity,annual rhythm of xylem production in relation to phenology and climatic factors and lignification pattern during xylogenesis in drum-stick tree (Moringa oleifera)

The interrelationship among seasonality of cambium, wood formation, cell size variation, lignification, tree phenology and climatic factors has been examined in Moringa oleifera, a tropical evergreen tree. The vascular cambium in Moringa is a storied with a distinct seasonal variation in its structure due to dimensional changes in rays. Though cambium remains active throughout the year it is sensitive to water availability. Peak cambial cell division and rate of xylem differentiation are influenced by average rainfall during the monsoon period. Cambial cell division reaches higher up in the tree trunk when it is supporting a high number of branches and leaves. Statistical analysis of cell size variation and climate factors revealed that xylem cell development is greatly influenced by rainfall and rarely by temperature. Lengths of fusiform initials and vessel elements are positively correlated. The pattern of lignification during xylogenesis shows that the vessels are the first element to develop lignified walls and ray cells are the last elements to become lignified. Fiber cell walls show more syringyl lignin, while the cell walls of other xylem elements are characterized by relatively more guaiacyl lignin units.     

Formation of successive cambia and stem anatomy of Sesuvium sesuvioides (Aizoaceae)

Mature stems of Sesuvium sesuvioides (Fenzl) Verdc. were found to be composed of successive rings of xylem alternating with phloem. Repeated periclinal divisions in the parenchyma outside the primary phloem gave rise to conjunctive tissue and the lateral meristem that differentiate into the vascular cambium on its inner side. After the formation of the vascular cambium, the lateral meristem external to it became indistinct as long as the cambium was functional. As the cambium ceased to divide, the lateral meristem again became apparent prior to the initiation of the next cambial ring. The cambium was exclusively composed of fusiform cambial cells with no rays. In the young saplings, the number of cambial cylinders in the axis varied from the apex to the base, indicating formation of several rings within the year. In each successive ring of the lateral meristem, small segments differentiated into the vascular cambium and gave rise to vessels, axial parenchyma, fibres and fibriform vessels towards the inside, and secondary phloem on the outer side. In the old stems, non‐functional phloem of the innermost rings was replaced by a new set of sieve tube elements formed by periclinal divisions in the cambial segments associated with the non‐functional phloem. In some places the cambial segments completely differentiate into derivatives leaving no cambial cells between the xylem and phloem. © 2008 The Linnean Society of London, Botanical Journal of the Linnean Society, 2008, 158 , 548–555.     

The Cambial Activity and on Which the Effects of Exogenous IAA in the Stem of Pinus sylvestris L.

The dormant cambial zone consisted of 5–6 cell layers in the main stem of Pinus sylvestris L. trees that were ca. I00 years old. Time of cambial reactivation was comparable at one (bottom) and 8 (top) meters above the ground. In spring, when the cambium reactivated, the number of cambial cells slightly increased and phloem cells were formed. The production of xylem cells followed 3–4 weeks later. The formation of xylem cells decreased, whereas that of phloem cells increased between late June and early July. Cambial reaction in 1-year-old cuttings that were debudded and treated apically with IAA in lanolin was similar to that in the ca. 100-year-old main stem. However, in debudded cuttings treated with plain lanolin, the number of cells in the carnbial zone decreased during the first week of culture, and only a few phloem cells were formed. Later, the fusiform cambial cells of the cambial zone were divided transversely and lost their typical morphology. It is proposed that some factor(s) from roots may stimulate the initiation of cambial cell division, because when the cambium reactivated, the number of cambial cells slightly increased in the ca. 100-year-old main stem, but decreased in the 1-year-old cuttings.     

Cambium Formation in Wounded Solanaceous Stems

When stems of certain Solanaceous species are wounded the internalphloem strands that lie near to the damaged surface become enlargedthrough cell division, and some of them develop a vascular cambium.This ‘internal cambium’ forms phloem inwards andxylem outwards; it is thus in inverted orientation. Such a structuredoes not occur in the normal stem of any Solanaceous plant.Further, during the regeneration of a cambial cylinder afterwounding, the regenerating cambium may turn inward to meet woundedinternal phloem strands. Where this happens, subsequent secondarythickening leads to the formation of thin tubes of cambium runningradially through the xylem and forming a limited amount of enclosedphloem which connects the internal and external phloem systemsof the stem. The jpositions in which these cambia arise are explicable bythe ‘gradient induction’ hypothesis of cambial regeneration,but not by the ‘free surfacfe’ and ‘cambialring’ theories.     

Reactivation of the Cambium in Aesculus hippocastanum L.: A Transmission Electron Microscope Study

Changes taking place during cambial reactivation in Aesculushippocastanum have been studied using transmission electronmicroscopy. Cytoplasmic activity in the form of vesicle productionby dictyosomes and endoplasmic reticulum, and coated vesicleformation at the plasmalemma, was observed in samples collectedin mid-Feb. The first cell divisions occurred 1 month later,in cells to the phloem side of the cambium, and were of twotypes: penclinal divisions producing new phloem precursors,and oblique anticlinal divisions in phloem mother cells formedat the end of the previous growing season producing putativecompanion cell/sieve element pairs. The fusiform initial wasidentified as the cell adjacent to the boundary-layer of parenchymacells and was the last cell to divide, 2 weeks after the firstdivisions in phloem precursors. For the next 4 weeks phloemcells only were produced The first new differentiating xylemelements were formed in the middle of Apr., following a surgein the rate of cell division by the initial aRd its derivativexylem mother cells. These were a mixture of developing fibresand vessel elements. Some of the boundary-layer cells were converteddirectly to vessel elements without any division taking place,while others were derived from daughter cells of the fusiforminitial produced following its reactivation. Aesculus hippocastanum L., cambium, dormancy, reactivation     

A rapid decrease in temperature induces latewood formation in artificially reactivated cambium of conifer stems

Background and Aims Latewood formation in conifers occurs during the later part of the growing season, when the cell division activity of the cambium declines. Changes in temperature might be important for wood formation in trees. Therefore, the effects of a rapid decrease in temperature on cellular morphology of tracheids were investigated in localized heating-induced cambial reactivation in Cryptomeria japonica trees and in Abies firma seedlings. Methods Electric heating tape and heating ribbon were wrapped on the stems of C. japonica trees and A. firma seedlings. Heating was discontinued when 11 or 12 and eight or nine radial files of differentiating and differentiated tracheids had been produced in C. japonica and A. firma stems, respectively. Tracheid diameter, cell wall thickness, percentage of cell wall area and percentage of lumen area were determined by image analysis of transverse sections and scanning electron microscopy. Key Results Localized heating induced earlier cambial reactivation and xylem differentiation in stems of C. japonica and A. firma as compared with non-heated stems. One week after cessation of heating, there were no obvious changes in the dimensions of the differentiating tracheids in the samples from adult C. japonica. In contrast, tracheids with a smaller diameter were observed in A. firma seedlings after 1 week of cessation of heating. Two or three weeks after cessation of heating, tracheids with reduced diameters and thickened cell walls were found. The results showed that the rapid decrease in temperature produced slender tracheids with obvious thickening of cell walls that resembled latewood cells. Conclusions The results suggest that a localized decrease in temperature of stems induces changes in the diameter and cell wall thickness of differentiating tracheids, indicating that cambium and its derivatives can respond directly to changes in temperature.     

Seasonal Changes in the Cambium of Trees. I. Sucrose Content in Thuja occidentalis

Cambium samples of Thuja occidentalis L. were collected at five different times, covering spring reactivation and early and late resting period, and used for sucrose determinations. Fragments of the different cell types - xylem ray, cambial initials, sieve-elements including phloem parenchyma cells, phloem ray - were dissected from freeze-dried radial sections and analyzed individually. Results show large differences in sucrose concentrations in the different cell types of the cambial layer. In addition, each cell type also shows seasonal fluctuations in sucrose content, whose amplitudes and patterns of variation appear specific for the particular cell type.     

The ontogeny of the vascular cambium inGinkgo biloba roots

Observation was made on early ontogeny of vascular cambium in the developing root ofGinkgo biloba L. After completion of root elongation, the vascular meristem gradually acquires cambial characteristics. Strips of the periclinal division of cells in transverse section are observed on the inner side of phloem when the primary xylem and phloem in the stele have been established. The strips are united into a continuous layer between phloem and xylem. In tangenital section, the procambium shows a homogeneous structure, which is initially composed of short cells with transverse end walls and subsequently, of long cells with tapering ends. Then, the procambium is organized into two systems of cells; axial strands of short cells with transverse end walls resulting from the sporadic transverse divisions of long cells, and long cells with tapering ends. Still later, the short cells are divided frequently in a trasverse plane exhibiting one or a few cells in width and several decades of cells in height, while the long cells are elongated. The frequency of transverse divisions of the short cells decreases in subsequent stages. Eventually, the short cells in axial strands are vertically separated from one another by the elongation of neighboring long cells and by the decrease in the frequency of transverse divisions of short cells themselves. Cambial initials occur in two forms; ray initials a few cells in height and one cell in width derived from the short cells, and fusiform initials with tapering ends derived from the long cells.