Linking the Evolution of Development of Stem Vascular System in Nyctaginaceae and Its Correlation to Habit and Species Diversification

Background: The presence of alternative patterns of secondary growth in stems of Nyctaginaceae has been known for a long time. Still, the interpretation of types of cambial variants are controversial. The knowledge on stem anatomical diversity in Nyctaginaceae, which is diverse also in habits, offers the unique opportunity not only to investigate the evolution of complex developments, but also to address how these anatomies shifted within habits and how the acquisition of novel cambial variants and habit transitions impacted the diversification of the family. Methods: We integrated developmental data with a phylogenetic framework to investigate the diversity and evolution of stem anatomy in Nyctaginaceae using phylogenetic comparative methods, reconstructing ancestral states, and examining whether anatomical shifts correspond to species diversification rate shifts in the family. Results: Two types of cambial variants, interxylary phloem and successive cambia, were recorded in Nyctaginaceae, which result from four different ontogenies. These ontogenetic trajectories depart from two distinct primary vascular structures (regular or polycyclic eustele) yet, they contain shared developmental stages which generate stem morphologies with deconstructed boundaries of morphological categories (continuum morphology). Unlike our a priori hypotheses, interxylary phloem is reconstructed as the ancestral character for the family, with three ontogenies characterized as successive cambia evolving in few taxa. Cambial variants are not contingent in habits, and their transitions are independent from species diversification.Conclusions: Our findings suggests that multiple developmental mechanisms, such as heterochrony and heterotopy generate the transitions between interxylary phloem and successive cambia. Intermediate between these two extremes are present in Nyctaginaceae, suggesting a continuum morphology across the family as a generator of anatomical diversity.

Formation of medullary phloem in Argyreia nervosa (Burm. f.) Bojer

Histologically, family Convolvulaceae is characterised by the presence of successive cambia, medullary (internal/intraxylary) and interxylary phloem in majority of the species, whereas some of the members are devoid of successive cambia and medullary bundles. The present study on Argyreia nervosa (Burm. f.) Bojer showed the presence of medullary bundles while internal phloem was absent during the primary growth. As the secondary growth progressed, successive cambia initiated from the pericyclic parenchyma. Development of medullary bundles began along with the regular protoxylem and protophloem while formation of intraxylary phloem was observed only after the initiation of secondary growth. Medullary/intraxylary sieve elements began to develop from the marginal pith cells. In thick stems, small segments of internal cambium initiated between the protoxylem and internal phloem. This internal cambium was functionally unidirectional and produced internal phloem centripetally. Developmental particulars are described in details along with its significance.

Development of successive cambia and structure of the secondary xylem in some members of the family Amaranthaceae

Young stems of Aerva javanica (Burm.f.) Juss. ex Schult., A. lanata (L.) Juss. ex Schult, A. monsonia Mart., A. sanguinolenta (L.) Blume, Alternanthera bettzickiana (Regel) G. Nicholson, A. philoxeroides (Mart.) Griseb., Gomphrena celosioides Mart., G. globosa L. and Telanthera ficoidea (L.) Moq., showed the renewal of small sectors of cambium by replacing with new segments. Therefore, the secondary phloem formed by earlier cambial segments form isolated islands of phloem enclosed within conjunctive tissues became embedded in the secondary xylem. As the stem grows older, complete ring of cambium is renewed; sometimes an anastomosing network of successive cambia may be seen due to the renewal of larger segments of the cambium. Renewal of the cambium takes place by repeated periclinal division in the parenchyma cells positioned outside to the phloem formed by the previous cambium. Functionally the cambium is bidirectional and exclusively composed of fusiform cambial cells. Differentiation of conducting elements of the secondary xylem and phloem remains restricted to the certain cambial cells while rest of the segments exclusively produce conjunctive cells. Accumulation of starch along with the presence of nuclei in the xylem fibers even after deposition of the secondary wall is consistent in all the species and it seems to be associated with the absence of rays in the secondary xylem and phloem of nine species from four genera. The significance of successive cambia, rayless xylem and nucleated xylem fibers were correlated with plant habit.

Stem anatomy at various developmental stages of secondary growth in Turbina corymbosa (Convolvulaceae)

Background – Population growth of lianas in the tropical forest is credited to their ability of CO2 sequestration and efficiency of the narrow stems to supply water required for the amount of foliage it bears. Turbina corymbosa (L.) Raf. (Convolvulaceae Juss.) is one of the fast-growing invasive species of scrambling woody lianas. It covers trees entirely within a short period to compete with above-ground resources (particularly sunlight). However, no information is available on how it manages to cope up with an increasing demand of water supply and mineral nutrients. What are the structural and developmental patterns adapted by this species to expand the stem diameter for efficient supply of below-ground resources? Therefore, our aim was to investigate the secondary growth patterns and structure of secondary xylem and phloem in T. corymbosa.Methods – Several samples of the stem with various diameters were studied using a histological method. Morphological and anatomical analyses were carried out using light microscopy.Key results – With the initiation of secondary growth, stems lose their circular outline rapidly due to unequal deposition of secondary xylem and formation of successive cambia. New successive cambia initiate from parenchymatous cells as small crescent-shaped fragments on asymmetric/opposite sides and result in a different stem conformation. Though several segments of successive cambia are formed, very few stem samples form complete cambium rings. The secondary xylem formed by successive cambia is diffuse porous with indistinct growth rings and is composed of both wide and narrow (fibriform) vessels, tracheids, fibres, axial and ray parenchyma cells. The secondary phloem consists of sieve tube elements, companion cells, axial and ray parenchyma cells. In fully grown plants, cambial action (internal cambium) occurrs between the intraxylary phloem and protoxylem and produces secondary xylem and phloem near the pith region.Conclusion – Structural alterations and unequal deposition of conducting elements, occurrence of intraxylary phloem and flattening of the stem are suggested to facilitate rapid growth of the plants by providing required minerals and nutrients. Internal cambium formed at the periphery of the pith is bidirectional and produces secondary xylem externally and intraxylary phloem internally. Continued development of intraxylary phloem from the internal cambium provides an additional path for rapid and safe translocation of photosynthates.