, the pipelines of the plant’s hydraulic system ( Cruiziat et al. 2002 ), remains an essential aspect of these studies ( e.g. , Pittermann et al. 2005 , 2006a ). Among paleobotanists, the question of the evolution of vascular systems is almost as old as the first observations of fossil plants ( e
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Anne-Laure Decombeix, Anaïs Boura and Alexandru M. F. Tomescu
Orly Lewis and Pavel Gregoric
ideas concerning the vascular system. 10 Given that the views of Praxagoras and Erasistratus have reached us only through later reports and citations, De spiritu constitutes the only first-hand testimony of the idea that artēriai contain only pneuma and no blood. Moreover, it is the earliest
S. van der Spoel
DENNIS A. BAKER
Ricinus phloem exudate. Planta 106: 131–140. Baker, D.A. and Allen, J.R.F. 1988. Auxin transport in the vascular system. In: Kutacek, M., Bandurski, R.S., and Krekule, J., eds. Physiology and biochemistry of auxins in plants. Academia, Prague, pp. 21–32. Baker, D.A. and Allen, J.R.F. 1992
, with results demanding a certain set of interpretations in order to allow pneuma its undisputed place in Galen's neurological system. That, surely, is Galen's intention. Galen's system of pneumatologies are dependent in large part on an explicable and understood vascular system, capable of anatomical
Males and females of the parasitic prosobranch Monogamus entopodia n. gen., n. sp. occur together within the same tube foot of Echinometra mathaei in the Gulf of Elat (= Gulf of 'Aqaba), Red Sea. The anatomy of the parasite and its relation to the host have been studied, and observations on its biology are given. The snout has a most unusual shape and is deeply inserted into the tube foot, but does not penetrate the hydro-vascular system, the tube foot component of which degenerates as a result of parasitization. Males and females of another species, Monogamus interspinea n. sp., described from a few Pacific Ocean localities, live close together on the aboral side of the test of Echinometra mathaei. Monogamus is probably related to Robillardia Smith, which lives in the rectum of Echinometra.
Keiko Kuroda and Ken Shimaji
The wound effects on cytodifferentiation in hardwood xylem were studied by means of periodical observation of wound tissue formation after a pin insertion into the stem of poplar. The mitotic reactivation of ray parenchyma cells was similar to that in conifers. These ray cell derivatives easily invaded other cells creating the impression of septate fibres. Conspicuous abnormalities were found in the differentiation of those fusiform cells which were situated in the zone of xylem mother cells at the time of wounding and those originating from cambial initials for several days after wounding. In the former zone, fusiform cells were prevented from differentiating into vessel elements after dividing transversely several times in the zone adjacent to the injury ; fusiform cells in the area extending several millimetres longitudinally were variously modified morphologically after the frequent transverse divisions in the xylem mother cell zone: they showed various transitional patterns from vessel element-like through tracheid-like, and axial parenchyma-cell-like to fibre-like. These observations suggest that the direction of cytodifferentiation is determined in the cambial initials or the neighbouring xylem mother cells, and is controlled by certain substances, which may change in concentration through the wounding stimulus, bringing about the modification in cytodifferentiation. Wound reaction of hardwood (i .e., woody dicotyledons) was thus completely different from the regeneration of vascular system in injured herbaceous dicotyledons.
Tommaso Anfodillo, Giai Petit and Alan Crivellaro
Within a tree the lumen of the xylem conduits varies widely (by at least 1 order of magnitude). Transversally in the stem conduits are smaller close to the pith and larger in the outermost rings. Axially (i.e. from petioles to roots) conduits widen from the stem apex downwards in the same tree ring. This axial variation is proposed as being the most efficient anatomical adjustment for stabilizing hydraulic path-length resistance with the progressive growth in height. The hydrodynamic (i.e. physical) constraint shapes the whole xylem conduits column in a very similar way in different species and environments. Our aim is to provide experimental evidence that the axial conduit widening is an ineluctable feature of the vascular system in plants. If evolution has favoured efficient distribution networks (i.e. total resistance is tree-size independent) the axial conduit widening can be predicted downwards along the stem. Indeed, in order to compensate for the increase in path length with growth in height the conduit size should scale as a power function of tree height with an exponent higher than 0.2. Similarly, this approach could be applied in branches and roots but due to the different lengths of the path roots-leaves the patterns of axial variations of conduit size might slightly deviate from the general widening trend. Finally, we emphasize the importance of sampling standardization with respect to tree height for correctly comparing the anatomical characteristics of different individuals.
Jonas Keiler, Stefan Richter and Christian S. Wirkner
Edited by R. Vonk
The emergence of king crabs from a hermit crab-like ancestor is one of the most curious events in decapod evolution. King crabs comprise two taxa, Lithodidae and Hapalogastridae, and while lithodids have formed the focus of various anatomical studies, the internal anatomy of hapalogastrids has never been studied although this group might represent a more ancestral morphological condition within king crabs than lithodids do. To better understand the evolutionary transformation of pagurid-like hermit crabs into king crabs, we studied the hemolymph vascular system and associated organs of representatives of Hapalogaster and present here the first micro-computer tomography data pertaining to the internal anatomy of hapalogastrids. Our results for Hapalogastridae are compared with existing and new data on Paguridae and Lithodidae and are discussed in the light of the process of carcinization or evolutionary transformation into a crab-like habitus. Hapalogaster resembles both pagurids on the one hand and lithodids on the other, not only with regard to external morphological characters but also in terms of certain internal anatomical features. In this and other respects, Hapalogaster represents an evolutionary intermediate form that connects pagurids and lithodids.
Brett A. Bergman, Edward G. Bobich, Stephen D. Davis, Yasuhiro Utsumi and Frank W. Ewers
A node is the point of attachment of the leaf to the stem of a plant; gaps associated with nodes have been viewed as discontinuities of the stem vascular system. We tested the hypothesis that the node/gap is a spring-like joint that impacts stem flexibility even well after the leaves have been shed, with some stems specialized for elongation and others for flexibility. Four-point bending tests were done using an Instron Mechanical Testing Device with the independent variable being the number of nodes in the stem segment and dependent variables being Modulus of Elasticity (MOE), Modulus of Rupture (MOR), and xylem density. Node anatomy was examined microscopically to assess structure and function. The stiffness of the stem was inversely proportional to the frequency of leaf nodes. Surprisingly, xylem density was inversely proportional to the frequency of leaf nodes in stems of adult trees. The tissue around nodes/gaps consisted of twisted and contorted cells that may be effective at absorbing compressive and tensile stresses. Because nodes behave as spring-like joints, the frequency of nodes relates to function, with some stems specialized for vertical expansion and others for light capture and damping of wind stress. The ultimate stems on a tree are the most bendable, which may allow the trees to avoid breakage.