Geophysical upheavals and evolutionary diversification of plant species in the Himalaya

The Himalaya is one of the youngest and the loftiest mountain chains of the world; it is also referred to as the water tower of Asia. The Himalayan region harbors nearly 10,000 plant species constituting approximately 2.5% of the global angiosperm diversity of which over 4,000 are endemics. The present-day Himalayan flora consists of an admixture of immigrant taxa and diversified species over the last 40 million years. The interesting questions about the Himalayan flora discussed here are: how did the Himalaya achieve high endemic plant diversity starting with immigrant taxa and what were the main drivers of this diversity? This contribution aims to answer these questions and raise some more. We review and analyze existing information from diverse areas of earth and climate sciences, palaeobiology and phytogeography to evolve a bio-chronological record of plant species divergence and evolution in the Himalaya. From the analysis we infer the effects of major environmental upheavals on plant diversity in the region. The understanding developed in the following discussion is based on the idea that Himalaya experienced at least five phases of major geophysical upheavals, namely: (i) mega-collision between India and Eurasian plates, (ii) tectonic uplift in phases and progressive landform elevation, (iii) onset of southwest (SW) Indian monsoon, (iv) spurring of arid conditions in Central Asia, and (v) cyclic phases of cooling and warming in the Quaternary. The geophysical upheavals that were potentially disrupting for the ecosystem stability had a key role in providing impetus for biological diversification. The upheavals produced new geophysical environments, new ecological niches, imposed physical and physiological isolation barriers, acted as natural selection sieves and led to the formation of new species. This contribution aims to develop a comprehensive understanding of the plant biodiversity profile of the Himalaya in the context of complex, interconnected and dynamic relationship between earth system processes, climate and plant diversity.

Mechanisms of Auxin Regulation of Structural and Physiological Polarity in Plants, Tissues, Cells and Embryos

New experiments and insights extend current concepts to yield four integrated hypotheses on polarization. (1) In tissues that lack polarity, as in meristems and proliferating callus, short-distance diffusive movement of auxin between spatially separate sources and sinks can gradually induce polar transport of auxin and associated proton currents in the direction of the initially diffusive movement. (2) The formation of auxin at the shoot tip, on which maintenance of the plant's polarity depends, occurs by hydrolysis of auxin precursors such as IAA-myo-inositol, which readily hydrolyses (even non-enzymically) at rates that increase with pH, so that higher pHs arising in leaf primordia through auxin-proton cotransport can promote local formation of auxin. (3) The long axes of cells and the secondary wall banding of tracheary elements tend respectively to develop parallel and perpendicular to the direction of polar transport of auxin through tissue, especially vascular tissue; these orientations are mediated by the bioelectric current associated with polar auxin transport, through orientation of cortical microtubules transverse to the current. (4) Initiation of polarity in zygotic embryos is controlled by the direction of auxin movement in the surrounding parental tissues; in lower plants the exoscopic embryo's polarity is similar to that of the surrounding tissues, but in seed plants the endoscopic embryo's polarity is inverted as a result of physiological isolation, localised auxin breakdown, and suspensor formation.

Cellular differentiation in small clumps of Lotus corniculatus callus

Lotus callus cultures were studied in an attempt to determine, at the cellular and subcellular levels, what morphological changes precede and accompany differentiation. Small clumps of homogenized callus were plated onto a medium containing benzyladenine, which was known to induce differentiation in this system. Initially callus was yellowish and consisted of large, vacuolated cells with deposits of starch. Marked changes occurred in these cells; peripheral and endogenous meristematic areas were initiated giving rise to shoots and either groups of tracheary elements or roots, respectively. Roots developed within 5 day s, while shoot apical meristems with leaf primordia formed by day 9. Many of the cells surrounding meristematic areas developed suberin lamellae in their walls, while others, both within and on the periphery of meristematic areas, accumulated phenolic substances. Cells within meristematic areas had large nuclei with prominent nucleoli, plastids with thylakoids but little or no starch, many mitochondria, and dictyosomes. Morphological observations tend to support the view that physiological isolation of tissue may precede differentiation.

The Spinal Nerves of the Dogfish (Scyliorhinus)

The locomotory musculature of dogfish is innervated by the segmental spinal nerves. The sensory and motor innervation of the abdominal musculature was studied in a preparation consisting of a strip of the abdominal body wall innervated by the ventral rami of the spinal nerves.Each ventral ramus consists of two separate nerve bundles which were found to be peripheral extensions of the dorsal and ventral spinal roots. Recordings from the sensory bundles showed that there are few sensory endings in the musculature and body wall of the dogfish. It was possible to differentiate between ephemeral responses produced by cutaneous free-nerve endings and prolonged discharges which were generated by more specialized sensory endings. In some details these endings were found to be unlike either muscle spindles or tendon organs. Further, skinning experiments suggested that these mechanoreceptors lay in the skin or the very outer layers of the myotome.Histological searching, together with physiological isolation of units, suggested that these receptors were the corpuscular endings distributed sparsely amongst subcutaneous tissue. These endings are apparently the same as those described by Wunderer (1908) in the fins of elasmobranchs.

IX — Studies in tunicate development Part IV—Asexual reproduction

Budding in Tunicates has long been a subject of considerable interest and controversy. The earlier investigations were concerned for the most part with the morphology of the budding processes, and many classifications based on the nature of the organs and tissues were evolved. More recently the emphasis, as represented by the series of papers by Brien, has been applied to the more fundamental problems of morphogenesis. The point of view of the present paper lies between the two just mentioned. The general biology of asexual reproduction has been studied throughout a much wider range of animals than in previous accounts, while in addition the nature and nutrition of the tissues of the bud have been investigated in each case but without analysis of the process by which the undifferentiated bud tissue develops the organization of the adult. Before any general discussion is attempted, a description of asexual reproduction will be given for each tunicate genus with that faculty. In each there is a concentration on three questions, the internal or external factors that induce this method of reproduction, the process by which the actual or physiological isolation of the parts from the whole is accomplished, and the nature of the tissues so isolated.