Phloem Formation Research Articles

The position of regenerating cambia: auxin/sucrose ratio and the gradient induction hypothesis.

To account for the positions in which vascular cambia regenerate in wound callus, a gradient induction hypothesis was proposed in 1961 in terms of gradients in 'some factor as yet unknown'. It now seems likely that the gradient is based on morphogen diffusion between source and sink on opposite sides of existing cambia, with morphogen diffusing into the adjoining wound callus. It is specifically proposed that there are two morphogens, auxin diffusing centrifugally and sucrose diffusing centripetally. The cambium then regenerates along a path where the ratio of auxin to sucrose concentration is similar to that at the original cambium, and its orientation (as regards xylem and phloem formation) is determined by the direction of the gradient in this ratio. These proposals are supported by published evidence on auxin and sucrose concentration gradients across the cambium, and on their sources, movements, and known effects on vascular differentiation. Simulations of the proposed positional control system predict patterns of cambial regeneration and orientation corresponding to those observed in four different types of wound and graft.

Seasonal activity of cambium in some tropical trees. 111. Salvadora persica L.

Seasonal activity of cambium has been investigated in Salvadora persica L., one of the commonest evergrenn tree of northern India, from March, 1974 to February, 1975 in the main stem and the branch. Secondary vascular cambium behaves abnormally on the xylem side, by producing xylem and phloem in succession, resulting in the formation of interxylary phloem. It has a storied (sitratified) cambium organization, comprising short fusiform and isodiametric ray initials. The fusiform initials show characteristically beaded cell walls due to the presence of primary pit fields and are uninucleate. These are highly vacuolate during the active period. The cambial activity starts in the month of March, reaches the peak in August and finally slows down in the main stem. In an young branch, however, it is initiated in March-April and reaches the peak in July and then declines till November. It once again showed an upward trend gradually. It appears that the initiation of the combial activity in this tree takes place by a hormone produced in the newly formed leaves, coupled with the high rainfall, enhanced relative humidity, optimum temperature, and short-day conditions. Size measurements for the fusiform initials showed considerable variations during the span of a year. A comparision of the organization of three types of phloem 'normal', 'included' and 'internal' occurring in this plant has also bean presented.

Zur Entwicklung der Kalium‐Mangelsymptome von Sommerraps

The development of potassium deficiency symptoms in summer rapeMacroscopic, microscopic and chemical investigations of summer rape plants, grown under glass at different levels of potassium, led to the following results.1. Marginal and intervenal chlorosis of sucessive leaves as well as withering and darkgreen colour can be considered as the early symptoms of potassium deficiency in rape.2. Chlorosis appears often (especially in cases of slight potassium deficiency) first of all in the middle leaves and later in the old leaves.3. With plants, recieving normal amounts of potassium, the highest concentration of potassium is found in the old leaves and the lowest in the young. Only in cases of acute deficiency is this reciprocal.4. The potassium deficiency symptom sequences, given in the literature, appear to be dubious, since the causal relationship, hitherto accepted, between the appearance of collapsed cortical tissue, the enlargement of parenchyma cells and secondary cambium activity does not exist.5. The cambium activity in the stalk with potassium deficiency and so the formation of xylem and phloem is greatly damaged. The formation of structural elements, such as sclerenchyma fibres and woody parenchyma cells, however, is especially affected by this deficiency.6. There are indications that stem necrosis, described as a typical potassium deficiency symptom, is primarily related to parasitic infection favoured by potassium deficiency.

The effects of 2-thiouracil on growth and metabolism in the root. Ι. Growth of excised root tissue

In this investigation isolated 2 to 4 mm subapical pea root segments were used to study the effect of 2-thiouracil on extension growth. 2-Thiouracil stimulated an increase in the lengths, fresh and dry weights, and polysaccharide material of the expanding segments, but only in the presence of a simple sugar. To produce the maximum stimulation it must be in contact with the tissue through out the entire cell expansion period. Uracil was an antagonist of 2-thiouracil stimulation. The analogue did not alter the morphological development of cells, xylem an d phloem formation being unaffected. Substances which are known to affect either growth (indole-3-acetic acid) or metabolism (D-threochloramphenicol and actinomycin D) were found to decrease or even abolish the stimulation caused by 2-thiouracil. 2-Thiouracil stimulation was also dependent on temperature, the maximum stimulation being observed at 25 to 30 °C.

A role of auxin in phloem regeneration in Coleus internodes

The physiology of regenerative phloem formation in the number 5 internode of Coleus blumei Benth. was studied by estimating quantitatively its extent in plants or plant parts wounded so as to sever one or more of the existing phloem bundles and then allowed to regenerate for a short period (usually 5 days). Removal of all leaves and buds from plants resulted in a marked reduction (to approximately one-quarter of the value of the intact controls) in phloem regeneration. Primary leaf removal also resulted in some, though a smaller, reduction, as did the one other treatment—removal of all distal organs—which involved removal of young primary leaves. The other patterns of shoot organ removal tried—all buds removed and all proximal leaves and buds removed—had no effect on phloem regeneration. Indoleacetic acid (IAA) applied in aqueous solution (2 mg/l) to plants from which all leaves and buds had been removed completely substituted for them in phloem regeneration. Applied in lanolin paste (0.1% and 1%), it evoked more phloem regeneration than attached organs themselves. Sucrose (2%) applied alone or together with IAA in aqueous solution had no effect. IAA (0.1%) applied in lanolin to excised internodes increased phloem regeneration in these, but the anomalous appearance of some of the wound sieve tubes indicated that sugar or other material may have become limiting for full differentiation. A relationship between the number of phloem bundles cut in wounding and the extent of phloem regeneration was shown. This and other observations were interpreted as indicating the involvement in phloem regeneration of a substance or substances leaking from the cut phloem. Existing evidence is interpreted to mean that auxin is limiting for the cell divisions which immediately precede phloem differentiation and that some other factor, presently unknown, may be limiting for differentiation when auxin is in sufficient supply for cell division. However, present evidence neither implies nor excludes the possible participation of auxin in differentiation itself.

STUDIES IN SYSTEMATIC BARK MORPHOLOGY

SummaryThe production of xylem and phloem sets up tangential stretching forces through the bark. In the living inner bark the resulting strains are mainly accommodated in Dipterocarpaceae by the formation of expansion tissue. The strain set up by a given annual girth increment decreases with increasing girth to a low steady amount at about 90 cm (3 ft) girth. Phloem formation in a mature tree only sets up small strains. These increase from zero at the cambium to a maximum at the surface. Many barks have more expansion tissue than this accounts for; the rest accommodates the strains set up by xylem formation. The tangential extent of the expansion tissue at a given distance from the cambium in any bark is in fact a measure of the ratio of the xylem and phloem growth rates. The three groups of Bark Types in Dipterocarpaceae with different amounts of expansion tissue (Whitmore, 1962a) are therefore groups with different xylem/ phloem ratios.Phloem growth rate determines periderm pattern and sometimes sloughing rate too. Fissure shape is controlled by periderm pattern and sloughing rate. The xylem growth rate is similar in different groups of dipterocarps. Thus the xylem/phloem ratio is an inverse measure of the phloem growth rate which cannot be measured directly. There is therefore a correlation between the xylem/phloem ratio and periderm pattern, sloughing and fissure shape.Similarities within the three groups of Bark Types in Dipterocarpaceae arise, therefore, from the dependence of periderm pattern, sloughing and fissure shape on the xylem/phloem ratio. Differences within the three groups arise from features, which are independent of the ratio: inter alia the phloem proliferation tissue, which probably varies phenotypically, and the sloughing mechanism.It follows that the xylem/phloem ratio is an important factor in the interpretation of diptero‐carp bark construction.There is no evidence of growth periodicity in the barks examined. Calculations show that the phloem is often conducting for more than 1 year to an observed maximum of 11.4 years; this is in marked contrast with most reports for temperate phloems.

Factors Influencing the Formation of Phloem and Heartwood Polyphenols Part II. The Availability of Stored and Translocated Carbohydrate. Part I: Holzforschung 14, 105 (1960).

Article Factors Influencing the Formation of Phloem and Heartwood Polyphenols Part II. The Availability of Stored and Translocated Carbohydrate. Part I: Holzforschung 14, 105 (1960). was published on January 1, 1962 in the journal Holzforschung (volume 16, issue 4).

Factors Influencing the Formation of Phloem and Heartwood Polyphenols. Part I Examination of a Specimen ofAcaciaWood Containing Tension Wood

Factors Influencing the Formation of Phloem and Heartwood Polyphenols. Part I Examination of a Specimen ofAcaciaWood Containing Tension Wood

Growth Studies in Forest Trees. III. Experiments with the Dendrograph on Fraxinus americana

1. Growth studies were made during the season of 1923 on Fraxinus americana L. by means of a dendrograph. Correlated histological investigations were made on the same tree. 2. No distortion of ring width is caused by the contact points or basal blocks of the dendrograph. 3. The desiccation of layers of phloem exposed in setting the instrument does not interfere with cambial activity. 4. The noon depression in diameter is greatest when the tree is in full leaf, supporting the supposition that this is brought about by the water relations of the tree. 5. A decided drop in temperature is usually followed by a decrease in or a cessation of growth. 6. The compensating variations before and after the growing period are closely correlated with precipitation. 7. The first xylem elements at the base of the trunk were formed simultaneously with the breaking of the leaf buds. Xylem formation ceased in this region about August 1. 8. Phloem formation began about three weeks after the first wood elements were formed, an...