Ultrastructural Observation of Changes of Polysaccharide Grains in Dormant Shoots of Eucommia ulmoides

Cytochemical and ultrastructural studies indicated that compound spherical bodies observed near the sieve plate in the sieve tube members during the period of cambial dormancy in the shoots of Eucommia ulmoides Oliv. were polysaccharide bodies different from the polysaccharide grains in other parenchyma cells in shoots, and were similar to cell walls in their staining properties. The compound spherical bodies occurred in the sieve elements during the period of cambial rest and disappeared in the sieve elements during the period of cambial quiescence and activity.     

杜仲休眠枝条中多糖颗粒变化的超微结构研究

在对杜仲（Ｅｕｃｏｍｍｉａ ｕｌｍｏｉｄｅｓ Ｏｌｉｖ．）形成层休眠期枝条的超微结构研究中发现,在筛管分子的筛板附近有一种复合球形体。细胞化学研究证明,这是一种多糖颗粒,但不同于枝条其他薄壁细胞中的多糖颗粒,而与细胞壁中的多糖相似。这种复合球形体只出现于形成层生理休眠期的筛分子中,而在被动休眠期和活动的筛分子中则看不到。     

EFFECT OF ISOLATION OF BARK ON CAMBIAL ACTIVITY AND DEVELOPMENT OF XYLEM AND PHLOEM IN TREMBLING ASPEN

Circular patches of bark were surgically isolated on the sides of trembling aspen (Populus tremuloides Michx.) trees at breast height at various times during the dormant and growing seasons. Subsequently, samples of wood and attached bark were taken from isolated and control sites to determine the effects of isolation of the bark on cambial activity and xylem and phloem development. In control trees cambial activity and xylem and phloem development occurred normally. Isolation of bark during the dormant season (in November, February, or March) did not prevent initiation of cambial activity and of phloem differentiation in spring but continued normal cambial activity and phloem developmented were prevent. Xylem differentiation was essentially prevented by isolation of tissues during the dormant season. The ultimate effect of isolation of the bark on the cambium, either during the dormant season or during the growing season, was subdivision of all fusiform cambial cells into strands of parenchymatous elements; the ultimate effect on the newly formed phloem was early death of the sieve elements. The most conspicuous effect of isolation of the bark after xylem differentiation had begun was the curtailment of secondary wall formation. Shortening of cells of the cambial region was reflected in the length of the vessel members which differentiated from such cells. These results indicate that normal cambial activity and xylem and phloem development require a supply of currently translocated regulatory substances from the shoots.     

Seasonal Changes of Cambial Activity and Phloem Development in Hevea brasiliensis

There is definitely seasonal activity in cambial growth in the trunk of Hevea brasi- liensis planted on Hainan Island and the cambial growth ceases during leaf-fail and leaf-absent period simultaneously with lower temperature and dries. The sieve elements founction for one-half year to two years. The variation in conducting-phloem thicknesses in a year is not significant and the conducting-phloem retains in about the same thickness during leaf-fall and leaf-absent period as that during the other periods in a year.     

THE CAMBIUM AND SEASONAL DEVELOPMENT OF THE PHLOEM IN ROBINIA PSEUDOACACIA

The cambium in black locust consists of several layers of cells at all times. Cambial reactivation (division) is preceded by a decrease in density of cambial cell protoplasts and cell wall thickening but not by cell enlargement. During the resumption of cambial activity, periclinal divisions occur throughout the cambial zone. Early divisions contribute largely to the phloem side. The period of greatest cambial activity coincides with early wood formation. Judged by numerous collections made during two seasons (October, 1960-October, 1962) the seasonal cycle of phloem development is as follows. Phloem differentiation begins in early April, ends in late September. The amount of phloem produced is quite variable (range: 1-10 bands of sieve elements per year). Cessation of function begins with the accumulation of definitive callose in the first-formed sieve elements and spreads to those more recently formed. By late November all but the last-formed sieve elements are collapsed. All sieve elements are collapsed by mid-winter and before the resumption of new phloem production in spring. Phloem differentiation precedes xylem differentiation by at least 1 week, and apparently functional sieve elements are present 3 weeks before new functional vessel elements. Xylem and phloem production ends simultaneously in most trees.     

Association of mycoplasma-like bodies with potato witches' broom disease from Scotland

Mycoplasma-like bodies were found by electron microscopy of sections of sieve tubes, both from shoots and roots of potato (Solanum tuberosum) plants affected by Scottish witches' broom disease and from graft-inoculated tomato shoots. The bodies were bounded by a unit membrane, contained ribosome-like material and mostly measured 200–800 nm in diameter. Most were oval in cross-section but some had lobes or slender protrusions. Some of the bodies were found in the mouths of sieve pores.     

THE SEASONAL CYCLE OF PHLOEM DEVELOPMENT IN JUNIPERUS CALIFORNICA

In Juniperus californica, all sieve cells of the previous season's phloem growth increment overwinter in a mature state. Initiation of cambial activity begins in early March and, by the end of March, the oldest sieve cells that overwintered lose their contents and die. By mid-April, even the youngest sieve cells of the previous season's growth increment have lost their contents. The period of greatest cambial activity begins in the last half of April and continues through May. With the slowing of cambial activity in June, callose begins to collect on the sieve areas of the first-formed sieve cells of the new increment. By July, the cambium and phloem are in a dormant state. Initiation of phloem production precedes that of the xylem by about 1 month. Production of new xylem and phloem ceases simultaneously in July.     

SEASONAL DEVELOPMENT OF THE SECONDARY PHLOEM IN ACER NEGUNDO

Functional sieve elements are present year-round in the secondary phloem of the trunk of Acer negundo L., the box elder tree. Judging from numerous collections made between May, 1962, and May, 1964, the seasonal cycle of phloem development is as follows: cambial activity and new phloem differentiation begin in late March or early April; xylem differentiation begins about a month later and is completed in most trees in late August. At the time of cessation of cambial activity most of the relatively wide sieve elements of the current season's increment are mature. However, numerous groups of narrow, immature sieve elements and companion cells located on the outer margin of the cambial zone do not reach maturity until fall and winter. By the time of cambial reactivation in spring, most, if not all, of these narrow elements are mature. Some of the sieve elements which reach maturity either shortly after cessation of cambial activity or during dormancy become non-functional within 6 weeks after resumption of cambial activity in spring, while others remain functional until mid-August. For the phloem increment of a given year, cessation of function begins in September with the accumulation of definitive callose on the sieve plates of the first-formed sieve elements and spreads to all but the last-formed ones by the end of December.     

Sieve Cell Differentiation; Accurate Determination by Polarized Light

Differentiating sieve cells can be qualitatively and quantitatively determined in white pine or other species of plants with phloem cells possessing nacreous primary walls or thickened secondary walls. Transverse sections from stained and unstained preparations of white pine examined in polarized light reveal a distinct zone of birefringent sieve cells situated between the cambial zone and layer of seasonal phloem parenchyma. The deposition of secondary walls in sieve cells in pine and their unequivocal recognition in polarized light presents a simple, effective means for detecting newly differentiated sieve cells and for quantitatively estimating their production during an experimental period.     

THE CAMBIUM AND SEASONAL DEVELOPMENT OF THE PHLOEM IN PYRUS MALUS

Evert , R. F. (U. Wisconsin, Madison.) The cambium and seasonal development of the phloem in Pyrus malus. Amer. Jour. Bot. 50(2): 149–159. Illus. 1963.—The cambium in apple consists of several layers of cells at all times, and practically all cambial cells divide periclinally one or more times before undergoing differentiation. The cambial initials do not seem to be in a uniform, uniseriate layer. Judged by collections made during 2 seasons (August, 1958–October, 1960), the seasonal cycle of phloem development is as follows. Early in April, cells in the outer margin of the cambial zone begin to differentiate into sieve elements. At approximately the same time, activity (division) commences throughout the cambial zone. By the end of July or early August, sieve-element differentiation is completed. Cessation of function begins in either late September or in October with the formation of definitive callose on the sieve areas of sieve elements in the outer margin of the functional phloem. By late November, all sieve elements are devoid of contents and most of their companion cells collapsed. Phloem differentiation precedes xylem differentiation by approximately a month and a half; xylem and phloem differentiation cease almost simultaneously; and fiber-sclereid development is coincident with the period of maximal xylem differentiation.     

Fluctuations in ribosomal RNA gene content and nucleolar activity in the cambial region of Abies balsamea (Pinaceae) shoots during reactivation

Tissue was collected from the vascular cambial region of 1-year-old balsam fir shoots over an 11-week period during which cambial reactivation occurred. The amount of rDNA (ribosomal RNA genes) relative to total genomic DNA was determined by quantitative slot blots for three trees, one of which showed a 3-week delay in reactivation. In addition, nucleolar activity was estimated by measuring nucleolar volume, number, and staining intensity. Relative rRNA gene content increased transiently prior to the onset of cambial cell periclinal division. Nucleolar volume also increased transiently, but 1–2 weeks prior to the maximal relative rDNA value. The increases in relative rDNA and nucleolar activity were delayed in the tree in which reactivation was late. We interpret these changes as reflecting the amplification and loss of genes encoding rRNA to facilitate cambial cell reactivation.     

Fine structure,pattern of division,and course of wound phloem in Coleus blumei

The wound phloem bridges which have developed six days after interrupting an internodal vascular bundle contain wound sieve-elements, companion cells, and phloem parenchyma cells. An analysis of the meristematic activity responding to the wounding clearly demonstrates that three consecutive divisions are prerequisite to the formation of phloem mother-cells. Companion cells are obligatory sister cells of wound sieve-elements, connected to the latter by specific plasmatic strands and provided with a dense protoplast. Six days after wounding most of the wound sieve-elements are still at a nucleate state of development, but already have characteristic P-protein bodies and plastids containing sieve-element starch. Their cytoplasmic differentiation corresponds to the changes recorded during maturation of ordinary sieve elements. Sieve-plate pores penetrate through preexisting parenchyma cell walls, only, and develop from primary pitfield-plasmodesmata. Wound sieve-elements do not connect to preexisting bundle sieve-elements, they open a new tier of young sieve elements produced by cambial activity.     

The Effect of Applied Growth Hormones on Cambial Division and the Differentiation of the Cambial Derivatives

When indole-3-acetic acid (IAA) is applied to woody shoots cambialdivision is stimulated and the cambial derivatives differentiateto produce xylem tissue. When gibberellic acid (GA) is applied,cambial division occurs but the resultant derivatives on thexylem side of the cambium remain undifferentiated. The relativelevels of applied IAA and GA are important in determining whethermainly xylem or phloem tissue is produced. High IAA/low GA concentrationsfavour xylem formation, whereas low IAA/high GA concentrationsfavour phloem production. The new phloem tissue produced asa result of hormone treatment is fully differentiated, containingsieve elements and sieve plates. IAA is important in promotingthe elongation of the cambial derivatives to produce xylem vesseland fibre elements, though in the case of xylem fibres appliedGA causes further elongation. IAA is an important factor indetermining vessel diameter in the ring-porous species Robiniapseudacacia, high levels of applied IAA giving wide springwood-typevessels and low levels giving narrow ‘summerwood’vessels.     

Anatomy of the vascular cambium of Acacia nilotica (L.) Del. var. telia Troup (Mimosaceae) in relation to age and season

the majority of fusiform initials are multinucleate, a few having as many as eight nuclei. Their length increases down the stem from the apex, attaining a maximum in the old trunk and declining slightly near the base. The width of the initials exhibits similar variation. In the main trunk, fusiform initials, relatively short at the time of cambial reactivation (April), elongate steadily until July. There is a sharp decline in August/September, the cell length recovering during the winter. Seasonal variation in cell width is inconsistent. Ray cell initials, on the other hand, do not vary much in size. They divide more frequently in the older stem, adding to the size of rays. In young shoots, short and uni- to biseriate rays are most abundant, whereas tall and multiseriate rays dominate the cambial surface in the trunk region throughout the year, with their minimum population in the early phase of cambial activity and the maximum during peak activity. The overall proportion of fusiform initials in the cambial cylinder initially increases with age, from young shoots towards the base, and later becomes more or less constant in the trunk region. Here it remains noticeably high during the active growth period and relatively low for the rest of the year.     

Anatomy of the vascular cambium of Acacia nilotica (L.) Del. var. telia Troup (Mimosaceae) in relation to age and season

the majority of fusiform initials are multinucleate, a few having as many as eight nuclei. Their length increases down the stem from the apex, attaining a maximum in the old trunk and declining slightly near the base. The width of the initials exhibits similar variation. In the main trunk, fusiform initials, relatively short at the time of cambial reactivation (April), elongate steadily until July. There is a sharp decline in August/September, the cell length recovering during the winter. Seasonal variation in cell width is inconsistent. Ray cell initials, on the other hand, do not vary much in size. They divide more frequently in the older stem, adding to the size of rays. In young shoots, short and uni- to biseriate rays are most abundant, whereas tall and multiseriate rays dominate the cambial surface in the trunk region throughout the year, with their minimum population in the early phase of cambial activity and the maximum during peak activity. The overall proportion of fusiform initials in the cambial cylinder initially increases with age, from young shoots towards the base, and later becomes more or less constant in the trunk region. Here it remains noticeably high during the active growth period and relatively low for the rest of the year.     

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.     

Seasonal Variations of Secondary Phloem Development in Pterocarya Stenoptera and Its Relation to Feeding of Kerria yunnanensis

Early in April of 1987, cells in an undifferentiated state which overwintered on the phloem side of the cambial zone in the branch of Pterocarya stenoptera began to differentiate into merebets of phloem. Cambium divided actively in mid-April and ceased to decide by early-Novembet. Five to eleven bands of fibers alternating with the bands of sieve tubes, companion cells and phloem parenchyma cells produced every year. By mid to late April, new xylem differentiation began. Phloem and xylem differentiation ceased almost simultaneously. Functional sieve tube elements were present all the year round in the phloem. During winter, most sieve tubes produced in the current year ceased functioning, leaving only the zone of functional sieve tube of several rows of cells in width with open pores in the sieve plates. These sieve tubes did not collapse until mid-May. In October, several rows of partially differentiated sieve elements appeared near the cambial zone. They still possessed nuclei. The companion cells had produced but no P-protein. They matured during April of the following year and collapsed by July to September. The life span of sieve elements extended for 8 months at the most. In winter, there were less functional sieve tubes in the branch. This may be one of the reasons that only few Kerria yunnanensis survive on the branch of Pterocarya stenoptera.     

Ultrastructure of P-protein inHevea brasiliensis during sieve-tube development and after wounding

Summary P-protein and the changes it undergoes after wounding of sieve tubes of secondary phloem in one- to two-year old shoots ofHevea brasiliensis has been studied using electron microscopy. The P-protein in the form of tubules with a diameter of 8–9 nm and a lumen of 2–2.5 nm occurred in differentiating sieve elements and appeared as compact bodies which consisted of small aggregates of the tubules. As the sieve elements matured, these P-protein bodies dispersed with a disaggregation of the tubules before they turned into striated fibrils, 10–11 nm in diameter. In wounding experiments, as the mature sieve elements collapsed after cutting, their striated P-protein converted into tubules. These tubules were the same in ultrastructure as the tubules in differentiating sieve elements and they often were arranged in crystalline aggregates.     

Structure and development of sieve cells in the secondary phloem of Larix decidua Mill. as related to function

Summary Only one or two layers of sieve cells of the previous year's phloem in lateral branches of Larix decidua persist as fully mature cells. Immature sieve cells or cambial derivatives that have not completed differentiation may also over-winter. Periclinal cell divisions of the vascular cambium were first observed by mid-April. During the short period of greatest cambium activity (mid-April to mid-May), the early phloem is laid down. Late phloem is formed over a much longer period, from mid-May to late September. Microautoradiography revealed that only mature sieve cells of the early phloem are involved in translocation of ¹⁴C assimilates in June. The fine structure of actively translocating sieve cells is described. The impact of structure on long-distance transport of assimilates is discussed.     

THE LATERAL MERISTEM AND ITS DERIVATIVES IN THE CORM OF ISOETES MURICATA

Corm tissue of Isoetes muricata Dur. was fixed in glutaraldehyde and postfixed in osmium tetroxide for electron microscopy. Very young secondary sieve elements can be distinguished from contiguous cambial cells by their distinctive plastids and by the presence of crystalline and/or fibrillar proteinaceous material in dilated cisternae of rough endoplasmic reticulum (ER). At maturity, the sieve elements are lined by the plasmalemma and a parietal, anastomosing network of smooth ER. Degenerate nuclei persist in all mature sieve elements. In addition, mature sieve elments contain plastids and mitochondria. Sieve-area pores are present in all walls. The lateral meristem of I. muricata consists of 2–3 layers of cells year-round. Judging from numerous collections made between October 1972 and July 1975, new sieve-element differentiation precedes cambial activity by about a month. Early in May, 1–2 cells immediately adjacent to already mature sieve elements differentiate directly into sieve elements without prior division. In early June, at about the time sieve-element differentiation is completed, cambial division begins. Division is sporadic, not uniform throughout the meristem. Dormancy callose accumulates in the secondary sieve elements in late October, and is removed in early May, at about the same time new sieve-element differentiation begins. Cells of the dormant cambium are characterized by the presence of numerous small vacuoles and large quantities of storage materials, including lipid droplets, starch grains, and tannin. By contrast, active cambial cells contain few large vacuoles with little or no tannin, and they have little storage material.