Vidya S. Patil

Development of intraxylary phloem and internal cambium in Ipomoea hederifolia (Convolvulaceae)

Abstract In Ipomoea hederifolia L. (Convolvulaceae), internal/intraxylary phloem originated as isolated strands from the procambially derived cells after the formation of protoxylem and protophloem. Bands of internal phloem were apparent in the sixth internode after the development of metacambium. In the relatively thick stems several small arcs/segments of internal cambium ensues from the parenchyma cells between the protoxylem and internal protophloem. Though all the segments were active, some of them (two of them located opposite to each other) were relatively more active. Bidirectional differentiation of these segments gave rise to secondary xylem centrifugally and secondary phloem centripetally, resulting inverted vascular bundles. Rest of the internal cambium segments were unidirectional and formed only secondary phloem centripetally. Like external vascular cambium, the internal cambium was non-storied. Structurally, secondary xylem and phloem was composed of axial and radial system in which rays were mostly uni- to biseriate. Secondary xylem produced by the internal cambium was more or less similar to the xylem formed by the external successive cambia. Secondary phloem produced by the internal cambium was composed of sieve tubes, companion cells, axial and ray parenchyma cells. Simple sieve plates of internal phloem were mostly arranged on transverse end walls in contrast to compound and obliquely placed sieve plates of external phloem formed by the successive cambia.

Wood Anatomy and the Development of Interxylary Phloem of Ipomoea hederifolia Linn. (Convolvulaceae)

In Ipomoea hederifolia Linn., stems increase in thickness by forming successive rings of cambia. With the increase in stem diameter, the first ring of cambium also gives rise to thin-walled parenchymatous islands along with thick-walled xylem derivatives to its inner side. The size of these islands increases (both radially and tangentially) gradually with the increase in stem diameter. In pencil-thick stems, that is, before the differentiation of a second ring of cambium, some of the parenchyma cells within these islands differentiate into interxylary phloem. Although all successive cambia forms secondary phloem continuously, simultaneous development of interxylary phloem was observed in the innermost successive ring of xylem. In the mature stems, thick-walled parenchyma cells formed at the beginning of secondary growth underwent dedifferentiation and led to the formation of phloem derivatives. Structurally, sieve tube elements showed both simple sieve plates on transverse to slightly oblique end walls and compound sieve plates on the oblique end walls with poorly developed lateral sieve areas. Isolated or groups of two to three sieve elements were noticed in the rays of secondary phloem. They possessed simple sieve plates with distinct companion cells at their corners. The length of these elements was more or less similar to that of ray parenchyma cells but their diameter was slightly less. Similarly, in the secondary xylem, perforated ray cells were noticed in the innermost xylem ring. They were larger than the adjacent ray cells and possessed oval to circular simple perforation plates. The structures of interxylary phloem, perforated ray cells, and ray sieve elements are described in detail.

Development of Intra- and Interxylary Secondary Phloem in Coccinia indica (Cucurbitaceae)

Stem anatomy and the development of intraxylary phloem were investigated in six to eight years old Coccinia indica L. (Cucurbitaceae). Secondary growth in the stems was achieved by the normal cambial activity. In the innermost part of the thicker stems, xylem parenchyma and pith cells dedifferentiated into meristematic cells at several points. In some of the wider rays, ray cells dedifferentiate and produce secondary xylem and phloem with different orientations and sometimes a complete bicollateral vascular bundle. The inner cambial segments of the bicollateral vascular bundle (of primary growth) maintained radial arrangement even in the mature stems but in most places the cambia were either inactive or showed very few cell divisions. Concomitant with the obliteration and collapse of inner phloem (of bicollateral vascular bundles), parenchyma cells encircling the phloem became meristematic forming a circular sheath of internal cambia. These internal cambia produce only intraxylary secondary phloem centripetally and do not produce any secondary xylem. In the stem, secondary xylem consisted mainly of axial parenchyma, small strands of thick-walled xylem derivatives, i.e. vessel elements and fibres embedded in parenchymatous ground mass, wide and tall rays along with exceptionally wide vessels characteristic of lianas. In thick stems, the axial parenchyma de-differentiated into meristem, which later re-differentiated into interxylary phloem. Fibre dimorphism and pseudo-vestured pits in the vessels are also reported.

Morpho-anatomy of Solanum pseudocapsicum

Morpho-anatomical features in leaves, stems and unripe fruits of Solanum pseudocapsicum L., Solanaceae, were investigated by histological methods. Anatomically the plant may be characterised by the presence of uniseriate trichomes, anomocytic stomata, calcium oxalate needles in leaves while presence of oval to circular compound starch grains, angular vessels, vertically upright, uni-biseriate rays and intraxylary phloem with differentiation of internal cambium abutting marginal pith cells and protoxylem in transverse view. Development of distinct internal cambium may be considered as a characteristic feature for S. pseudocapsicum. Intraxylary secondary phloem was composed of sieve tube elements, companion cells and axial parenchyma cells.

Development of included phloem of Calycopteris floribunda Lamk. (Combretaceae)

Abstract Structure and development of included phloem was investigated in the stems of Calycopteris floribunda Lamk., of the Combretaceae. After the definite period of cambial activity, cells in the middle of the cambial zone began to differentiate into thin walled cambial derivatives which separated the cambium into outer and inner cambial segment at certain places. Rest of the cambium along with separated outer segment remained functionally active while inner segment became temporarily nonfunctional. Original circular outline of the cambial cylinder was restored by joining of outer segment with existing one whereas inner cambial segment got embedded resulting in production of an islands of included phloem in the secondary xylem. This process was repeated several times resulting in a number of phloem islands surrounded within thick walled secondary xylem. Differentiation of phloem elements was initiated only after the formation of thick walled xylem derivatives from the outer cambial segment. The segments of the cambium producing the phloem island remained active for fairly long time. Sieve tube elements of the phloem islands situated deep inside the older stem became non-functional and underwent obliteration after heavy accumulation of callose. Secondary xylem was diffuse porous with indistinct growth rings and composed of vessels (both wider and fibriform vessels), nucleated xylem fibers, axial and ray parenchyma. Perforated ray cells were also encountered frequently in all the samples studied. Structure and development of included phloem is described in detail and possible significance of nucleated fibers and perforated ray cells is discussed.

Structure And Development Of Internal Phloem In Solanum Pseudocapsicum (Solanaceae)

The development of internal phloem in the Jerusalem cherry, Solanum pseudocapsicum L. (Solanaceae), was studied in young and mature stems. The early presence of primary internal phloem is succeeded by the development of secondary internal phloem from an internal cambium situated between the protoxylem and primary internal phloem. In the second and third visible internodes of the young stem, procambial derivatives begin to differentiate as discrete strands of internal protophloem in a perimedullary position prior to the differentiation of protoxylem and external protophloem. In 6–8 mm diameter stems, sieve elements of the internal phloem become non-conducting, begin to collapse, and undergo obliteration. In 15–20 mm diameter stems internal cambium is initiated from the parenchyma cells situated between the protoxylem and primary internal phloem. The development of internal phloem and an internal cambium in S. pseudocapsicum is compared with that in other taxa. There seems to be a gradual variation in the origin of an internal cambium from either remnants of the procambium or dedifferentiation of peripheral pith cells across dicotyledons with an internal cambium.

Multiple cambia and secondary xylem of Ipomoea pes-caprae (L.) R. Br. (Convolvulaceae)

Abstract Structure of secondary xylem and pattern of secondary thickening in climbing species are different from those in self-supporting plants. In many climbing species, stem diameter increases by forming more than one ring of cambium (referred to as multiple/successive cambia), while their secondary xylem usually contains abundant parenchyma, large vessels and wide rays. In beach morning glory (Ipomoea pes-caprae (L.) R. Br., Convolvulaceae), stem thickness increases by forming multiple rings of cambia. After a short period of normal secondary growth, the first successive cambium ensues from the pericyclic parenchyma. Thereafter, subsequent cambial rings originate from parenchyma cells produced initially by the previous cambium. In stems that are 15–20 mm thick, parenchymal cells produced by the initial activity of the previous cambia become meristematic and form small arcs of functionally inverse cambia that produce secondary xylem centrifugally and secondary phloem centripetally. Unequal production of secondary xylem by these cambia gives the stem various shapes other than cylindrical. Besides successive cambia, some cambial variants also develop in the stem which are: (1) irregularly distributed patches of thin-walled xylem parenchyma becoming meristematic and differentiating into interxylary phloem islands; (2) xylem ray cells acquiring meristematic character and behaving like cambium (referred to as ray cambium); and (3) in thick stems, internal cambium deriving from marginal pith cells, which are functionally bidirectional and producing secondary xylem centripetally and phloem centrifugally. Structure and development of successive cambia, ray cambia and internal cambium are discussed here.

Development of Cambial Variant in Sesuvium portulacastrum L. (aizoaceae)

Abstract Stems in Sesuvium portulacastrum L. (Aizoaceae) increase in thickness by forming successive rings of cambia that formed concentric rings of xylem alternating with phloem. The cambium is semi-storied and exclusively composed of vertically elongated fusiform initials while cambial rays were absent in the early part of the secondary growth. In the older stems some of the fusiform cambial cells undergo further division and develop into vertically upright rays. The first ring of cambium formed thick walled lignified elements centripetally and phloem centrifugally for a definite period. A second ring of cambium is developed from axial parenchyma cells located outside the phloem produced by the previous cambium. Prior to the complete differentiation of new cambial ring, these parenchyma cells divide bi-directionally to form additional parenchyma cells (referred here as secondary cortex) to the outer and inner side. Cells formed outside to the new ring formed secondary cortex, which served as a site for the future cambium. Inner cells differentiated later on into conjunctive tissue between two successive rings. In the newly formed cambium, alternate small segments of the cambium gave rise to conducting elements of xylem towards the center and phloem towards the periphery while the other segments formed thin walled parenchyma on both the sides. Secondary xylem was composed of vessel elements, fiber tracheids and libriform fibers with nuclei whereas secondary phloem consisted of sieve tube elements, companion cells and parenchyma with no rays in the young stem.

Cambial activity in the young branches and peduncles of couroupita guianensis (lecythidaceae)

Peduncles of Couroupita guianensis Aubl. undergo extensive secondary growth, which is a rare and unexplored feature so far. In the present investigation seasonal behaviour of vascular cambium was studied in fruit-bearing peduncles and compared with the vegetative branches of similar diameter. In peduncles, the cambium remained active throughout the year. The number of cambium cells and differentiating xylem cells increased from May and reached a maximum in July-August. Although cambial growth occurred throughout the year, it was relatively sluggish in February despite the development of new leaves and ongoing extension growth. In contrast, cambial cell division in young branches initiated in February, peaked in the same months as peduncle cambium while cambial cell division and differentiation of xylem remained suspended from October to January. Cessation of cambial cell division in the branches during this period may be correlated with the presence of mature leaves. In both (branches and peduncle), rapid cell division and increase in the number of differentiating xylem elements in April-May is positively correlated with the development of flower buds and new leaves. The present anatomical investigation revealed that cambial activity in both peduncle and vegetative branches are independent of phenology and climatic conditions. In conclusion, we believe that variations in the number of differentiating cambium derivatives in peduncles benefits from a dual source of growth hormone supply, i.e. from developing new leaves and flower buds.