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.     

SEASONAL DEVELOPMENT OF THE SECONDARY PHLOEM IN PINUS

Functional sieve cells are present at all times in the secondary phloem of Pinus banksiana Lamb., P. resinosa Ait., and P. strobus L. With regard to a given year's growth increment, all but the last-formed sieve cells (2-4 layers) cease functioning the same season they are derived from the cambium. The former overwinter and remain functional until new sieve cells differentiate in spring. Toward the end of March undifferentiated cells in the outer margin of the cambial zone begin to differentiate into sieve cells. About a week later, cambial activity (cell division) commences. All early phloem is produced by early May before new xylem differentiation begins. Most sieve cells are differentiated by late August, but a few not until late September. Cessation of function begins in late May or June with formation of definitive callose on sieve areas of the sieve cells which overwintered and continues slowly to sieve cells of the current season's early phloem. By mid-December all but the last-formed sieve cells (i.e., those which will overwinter in a functional state) are devoid of contents. Phloem differentiation precedes xylem differentiation by approximately 1 1/2 months. Xylem and phloem production cease more or less simultaneously in August, xylem and phloem differentiation in September.     

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.     

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.     

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.     

Seasonal development of phloem in Siberian larch stems

The seasonal development of phloem in the stems of Siberian larch (Larix sibirica Ldb.) was studied over two seasons on 50–60-year-old trees growing in a natural stand in the Siberian forest-steppe zone. Trees at the age of 20–25 years were used to study metabolites in differentiating and mature phloem elements, cambial zone, and radially growing xylem cells in the periods of early and late wood formation. The development of the current-year phloem in the stems of 50–60-year-old trees started, depending on climatic conditions, in the second-third decades of May, 10–20 days before the xylem formation, and ended together with the shoot growth cessation in late July. Monitoring of the seasonal activity of cambium producing phloem sieve cells and the duration of their differentiation compared to the xylem derivatives in the cambium demonstrated that the top production of phloem and xylem cells could coincide or not coincide during the season, while their differentiation activity was always in antiphase. Sieve cells in the early phloem are separated from those in the late phloem by a layer of tannin-containing cells, which are formed in the period when late xylem formation starts. The starch content in the structural elements of phloem depends on the state of annual xylem layer development. The content of low molecular weight carbohydrates, amino acids, organic acids, and phenols in phloem cells, cambial zone, and xylem derivatives of the cambium depends on the cell type and developmental stage as well as on the type of forming wood (early or late) differing by the cell wall parameters and, hence, by the requirement for assimilates. Significant differences in the dynamics of substances per dry weight and cell were observed during cell development.     

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.     

Periodicity of Cambium Activity and Seasonal Changes of the Secondary Phloem in Two Species of Dalbergia

Periodicity of cambium activity, seasonal changes of the secondary phloem and longevity of sieve tube in main trunk of Dalbergia balansae Prain and in the twig of D. szemaoensis Prain were observed. The results are as follows: 1. All cambia fall under the category of storied type. 2. In D. balansae cambial activity begins in late April and ends in early November. Phloem differentiation is completed by early November. Xylem differentiation ceases in December. In D. szemaoensis cambial activity continues from mid-April to late October. Phloem and xylem differentiation ceases by late November. 3. The width of functional phloem zone is maximal (400—600 μm) in autumn and minimal (200—370 μm) in February to April. In overwintering, functional sieve tube elements contain P-protein, and the pores of sieve plate are open. It could be one of the reasons that these two species are promising host trees of Kerria yunnanensis during winter. 4. The longevity of sieve tubes in D. balansae and D. szemaoensis last 8—12 months and 9—11 months respectively. 5. During dormancy of cambium, the parenchyma cells of the secondary phloem contain large quantities of starch grains and calcium oxalate crystals, which decrease as cambium becomes active and remain little or even non visualized in summer.     

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.     

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.     

Seasonal variations in the cambial anatomy of Tectona grandis (Verbenaceae)

A study of seasonal activity of the cambium in Tectona grandis L. f. has shown that the activity initiates in the first week of June, reaches a peak in July and then slowly declines. The length of fusiform cambial initials undergo considerable variations during the activity and dormancy of the cambium. The initiation of cambial activity is closely associated with the opening of the dormant foliar buds in the first week of May. Cambium is more active with high numbers of immature xylem and phloem elements from July to September when the trees are with mature foliage and flowers and dormant from January to April when leaves dry and defoliation takes place. The differentiation of xylem and phloem starts simultaneously and the number of their immature elements reach the maximum in July.     

INFLUENCE OF PHLOEM BLOCKAGE ON CAMBIAL GROWTH OF SUGAR MAPLE

Circular patches of bark were surgically isolated on the sides of sugar maple (Acer saccharum Marsh.) 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 sites cambial activity and xylem and phloem development occurred normally. Isolation of bark during the dormant season (in November, February, or March) prevented initiation of cambial activity and xylem and phloem development in isolated areas of half of the trees. Varying degrees of cambial activity (periclinal divisions) occurred in the remaining isolated areas, but normal cambial activity and xylem and phloem development were prevented. Isolation of bark after initiation of cambial activity and phloem differentiation, but prior to initiation of xylem differentiation, resulted in the formation of very narrow xylem and phloem increments with atypically short vessel members and sieve-tube members, respectively. The xylem increments consisted primarily of parenchyma cells. Isolation of bark after initiation of xylem differentiation resulted in curtailment of secondary wall formation in the last-formed part of many increments. The last-formed vessel members of all these xylem increments were atypically short. Similarly, the last formed sieve-tube members of corresponding phloem increments were atypically short. The atypically short cells in the xylem and phloem of isolated areas reflected the effect of isolation on the cambial region, viz., the subdivision of all fusiform cells into strands of cells. Ultimately, the strands of short fusiform cells lapsed into maturity, leaving only strands of parenchymatous elements between xylem and phloem.     

Seasonal Changes in the Structure of the Secondary Phloem of Grewia tiliaefolia, a Deciduous Tree from India

Structure of the secondary phloem of Grewia tillaefolia Roxb.was studied in samples of bark collected at monthly intervalsfrom forest populations of Gujarat in western India. The secondaryphloem in this species is vertically storied and the axial elementsoccur as alternate tangential bands of fibres and sieve elementsproduced in succession. On average, two to four bands of fibresand corresponding bands of sieve elements are produced in ayear. The sieve elements function for more than one season anddifferent phloem increments are separated by terminal zonesmade up of very narrow sieve elements which mature just beforeand immediately after the period of dormancy. The tree becomesleafless about eight to ten weeks preceding the spring equinox.Cambial activity, phloem differentiation and phloem functionare suspended during this period. Differentiation of phloembegins after bud break which occurs in April, and continuesuntil January, but most of the phloem is produced between Julyand September when the rainy season is well advanced. The widthof the conducting zone is maximal at the end of the period ofgrowth when the tree is in full leaf. Inactivation of sieveelements, apparently by callose plugging the sieve plates, beginswith leaf abscission. The sieve elements produced in the precedingseason, just before dormancy is imposed resume function in thefollowing growing season and the older elements die. Companioncells and axial parenchyma cells surrounding sieve elementsappear to have s significant role during senescence of the conductingelements. The development and activity of the secondary phloemseem to be related to other developmental phenomena occurringwithin the tree.     

Changes of Peroxidase and Estease Isozymes During Periodicity of Cambial Activity in Broussonetia papyrifera (L.) Vent.

Methods of sampling and sections preparaction were the same as reported previously. Except that sampling was made at monthly intervals between May 20 and July 30, then at 7–14 day-intervals between July 30 and October 14, and then at monthly intervals between October 14 and March 25 in the next year. The stored starch in various tissues was stained with PAS reaction. During active period of cambium in Broussonetia papyrifera after July 30, the cell layers of immature xylem and phloem decreased progressively, and the formation of mature xylem and phloem increased rapidly. The formation of late wood started early in August, formation of xylem ceased after September 5, followed by ceasation of phloem formation about 1.5 months later. Increasing and decreasing of stored starch were closely related to the periodicity of cambial activity during the year. Starch grains decreased progressively after cambial activity was resumed in early spring until they disappeared in all the stem tissues. Then, starch accumulated progressively again after cambial activity slowed down, particularly after the ceasation of xylem formation. However, after the formation of phloem had ceased, the stored starch once again disappeared progressively until the end of December, and accumulated again. Such changes might be related to the transition of cambium activity involving two periods of dormancy.     

Periodicity of Cambial Activity and Changes of Starch Content in Broussonetia papyrifera

Methods of sampling and sections preparaction were the same as reported previously. Except that sampling was made at monthly intervals between May 20 and July 30, then at 7–14 day-intervals between July 30 and October 14, and then at monthly intervals between October 14 and March 25 in the next year. The stored starch in various tissues was stained with PAS reaction. During active period of cambium in Broussonetia papyrifera after July 30, the cell layers of immature xylem and phloem decreased progressively, and the formation of mature xylem and phloem increased rapidly. The formation of late wood started early in August, formation of xylem ceased after September 5, followed by ceasation of phloem formation about 1.5 months later. Increasing and decreasing of stored starch were closely related to the periodicity of cambial activity during the year. Starch grains decreased progressively after cambial activity was resumed in early spring until they disappeared in all the stem tissues. Then, starch accumulated progressively again after cambial activity slowed down, particularly after the ceasation of xylem formation. However, after the formation of phloem had ceased, the stored starch once again disappeared progressively until the end of December, and accumulated again. Such changes might be related to the transition of cambium activity involving two periods of dormancy.     

构树形成层的活动周期及其淀粉贮量的变化

在构树（Ｂｒｏｕｓｓｏｎｅｔｉａ ｐａｐｙｒｉｆｅｒａ （Ｌ．） Ｖｅｎｔ．）形成层活动周期中,每年７月末以后,未成熟的木质部和韧皮部逐渐减少,成熟的木质部和韧皮部急剧增多。８月初开始分化晚材。进入９月后木质部的形成逐渐停止,而一个半月以后才停止形成韧皮部。淀粉贮量的消长与形成层的活动周期有很强的相关关系。早春形成层恢复活动后,淀粉贮量逐渐减少直至消失。尔后,形成层活动减慢,特别是木质部分化停止后,淀粉又开始积累。当韧皮部分化也停止后,淀粉又消失,直至翌年１月才重新积累,这似乎与两个休眠期的转化有关     

The Resumption of Cambial Activity and the Change of Peroxidase Isozymes in Broussonetia papyrifera

Five Broussonetia papyrifera (L.) Vent. trees were selected in a natural stand located on the campus of Peking University, Beijing, China. The trees were ca. 5-6 years old, 3-4 m tall,and had diameters of about 3 cm measured 1.2 m above ground level. They were samplied at monthly intervals between January 28 and March 25, then at ten-day intervals between March 25 and May 20,1991. On each occasion, one 3-year-old shoot was cut from the tree. Two blocks (about 1 cm ×1 cm) contained peridern,phloem,cambium and wood with more than one annual ring were cut from every shoot,fixed in FAA,and then were prepared for anatomical studies. And on each occasion,7 layers of tissues (from periderm to mature xylem)were scraped off from the shoots and 100 mg of separate tissues were randomly extracted in 0.1 ml of 20% sucrose. The extracts were used for isoelectric-focusing in polyacrylamide gel slabs (85 mm × 60 mm × 1 mm). Benziding and odianisidine was used as substrate. After electrophoresis the gel slabs were placed in the substrate buffer until the isozyme bands were visible. Owing to the ring-porous structure of the wood of Broussonetia papyrifera, the cambial activity was comparable with that in the most ring-porous dicots. The cambium activity started about ten days before bud sprouting. On April 4,the dormant cambial zone consisted of ca. 4 cell layers. The trees did not sprout until April 16,but ca. 2 cell layers of immature xylem and phloem were formed concomitantly. Ten days later, 8-9 cell layers of xylem and ca. 5 cell layers of phloem were formed. The formation of immature phloem cells continued to increase slowly between April 4 and May 20, whereas that of immature xylem cells increased rapidly between April 4 and April 26,and then decreased between April 26 and May 20. It was suggested that differentiation of immature xylem into mature xylem lasted ca. 10 days,whereas that of immature phloem into mature one lasted ca. 20 days. There were totally 6 peroxidase isozyme bands in dormant cambial region and functional phloem. Variation of zymogram in cambial region occurred before cambial activity activated which is followed by more or less minor changes of bands in all other tissues. These indicated that several significant changes were related to the level of endogenous IAA and differentiation of vascular tissues.     

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.     

Seasonal development of phloem in scots pine stems

The formation of phloem was studied for two years in stems of 50 to 60 year old trees of Scots pine (Pinus sylvestris L.) growing in nature. The development of phloem of the current year begins 10 to 20 days before the xylem formation and is completed with the termination of shoot growth in the end of June. Observations over the seasonal activity of cambium producing sieve cells of phloem and duration of their differentiation as compared to the xylem derivatives of cambium have shown that the maxima of formation of phloem and xylem cells could coincide or not coincide by season, while the activities of their differentiation were always in antiphase. The sieve cells of early phloem were separated from those of late phloem by a layer of tannin-containing cells, which are formed simultaneously with the formation of late xylem cells by the cambium. Seasonal dynamics of accumulation of starch grain in structural elements of the phloem is related to the xylem development. The content of metabolites in differentiating and mature phloem elements, in the cambium zone, and in the xylem cells growing in the radial direction depended on cell specificity, stage of their development, and type of forming wood, early or late, which differ in the cell wall parameters and, hence, requirement of assimilates. Significant differences were described between the content of low molecular weigh carbohydrates, amino acids, organic acids, and phenol compounds using two methods of calculation: per dry weight and per cell.     

Seasonal Variation in Radial Growth and Phloem Activity of Terminalia ivorensis A. Chev

Radial growth in five Terminalia ivorensis trees has been recordedfrom dendrometer reading for a period of 12 months. The durationof the growing season was 7–9 months. Variation in annualradial increment between individual trees was observed to bedue both to differences in the length of the growing seasonand the rate of growth during that period. Seasonal changesin the diameter of sieve elements, and the extent of callosedeposition on the sieve plates have also been investigated.Sieve element diameters were smallest in the dry season, possiblybecause of shrinkage. The width of phloem tissue showing definitivecallose was fairly constant throughout the year, but the zonewith open pores on the sieve plates changed, being widest inSeptember, and narrowest in March when the trees were almostbare. There were two peaks of cambial activity, indicated byan increase in width of the ‘open pore zone’, onein April at the time of bud break, and a second in September. The sugar concentration of the phloem exudate obtained fromsmall cuts into the bark of the trees varied throughout theyear. Concentrations were highest in March, during the dry season,and lowest in May, when the young leaves were expanding. Terminalia ivorensis A. Chev., tropical timber tree, radial growth, callose, phloem exudate, phloem activity