Medullary structure differences in Gelidium and Pterocladia (Gelidiales, Rhodophyceae): Taxonomic meaning

Study of the apical portions of vegetative thalli of 21 species of the genera Gelidium and Pterocladia (Gelidiales-Rhodophyta) revealed the existence of six patterns of medullary structure. Each pattern is characterized by the organization of the medullary tissue and the changes in cellular dimensions from apex to subapex. These characters can be used to separate groups of species within each genus.     

Photosynthesis and root growth in Spartina alterniflora in relation to root zone aeration

Spartina alterniflora Lois. is a dominant species growing in intermediate and saline marshes of the US Gulf coast and Atlantic coastal marshes. S. alterniflora plants were subjected to a range of soil redox potential (Eh) conditions representing a well aerated to reduced conditions in a rhizotron system under controlled environmental conditions. The low soil Eh resulted in inhibition of root elongation shortly after treatment initiation. Root elongation was reduced as soil Eh approached values below ca. +350 mV. Substantial decrease in root elongation was noted when soil Eh fell below +200 mV. Generally, net photosynthetic rate (PN) decreased as soil Eh was reduced, with substantial reductions in PN found when Eh approached negative values. Average PN was reduced to 87, 64, and 44% of control under +340, +245, and -180 mV treatments, respectively. The reductions in root elongation and PN in response to low soil Eh indicated the adverse effects of low soil Eh on plant functioning and the need for periods of soil aeration that allow plants to resume normal functioning. Thus periods of drainage allowing soil aeration during the growing season appear to be critical to S. alterniflora by providing favorable conditions for root growth and gas exchange with important implications for plant carbon fixation. This revised version was published online in June 2006 with corrections to the Cover Date.     

What is the optimal number of leaves when measuring leaf area of tree species in a forest community?

植物形态性状叶面积简单易测, 能够反映植物对环境的适应与响应, 指示生态系统的功能与过程。在野外测定叶面积时, 叶片取样数量往往采用约定俗成的10-20片, 但到底采集多少叶片才是最优和最具代表性, 却少有探究。该研究以浙江金华山常绿落叶阔叶混交林的优势树种木荷(Schima superba)与枫香树(Liquidambar formosana)为研究对象, 通过对5个胸径等级植株和每个植株6个方位开展大批量叶片取样(>2 500个), 分析两个树种的叶面积变异特征, 探讨叶片取样数量为多少才能最代表该森林类型的叶片大小性状规律。结果表明, 常绿乔木木荷平均叶面积与变幅均小于落叶乔木枫香树。木荷叶面积与胸径无显著相关性, 而枫香树叶面积与胸径有较显著相关性, 但两个树种均在中胸径等级(15-20 cm)差异不显著; 两个树种的叶面积与采样方位无显著相关性, 但在东、西和底部的差异不显著。因此, 综合考虑代表性与野外可操作性, 叶片采集首选中胸径成树的底部叶片。随机抽样统计可知, 树木叶面积测定的最适叶片采集数量因物种而异, 木荷的最适叶片采集数量为40, 而枫香树最少为170片。因此, 在叶面积测定时, 叶片采集的数量应该不能只局限在10-20片, 在人力、物力和时间等条件允许的情况下, 应该尽可能多地测定较多叶片的叶面积。     

Deconstructing the long-standing a priori assumption that serial homology generally involves ancestral similarity followed by anatomical divergence

It has long been assumed that serial homologues are ancestrally similar—polysomerism resulting from a “duplication” or “repetition” of forms—and then often diverge—anisomerism, for example, as they become adapted to perform different tasks as is the case with the forelimb and hind limbs of humans. However, such an assumption, with crucial implications for comparative, evolutionary, and developmental biology, and for evolutionary developmental biology, has in general not really been tested by a broad analysis of the available empirical data. Perhaps not surprisingly, more recent anatomical comparisons, as well as molecular knowledge of how, for example, serial appendicular structures are patterned along with different anteroposterior regions of the body axis of bilateral animals, and how “homologous” patterning domains do not necessarily mark “homologous” morphological domains, are putting in question this paradigm. In fact, apart from showing that many so-called “serial homologues” might not be similar at all, recent works have shown that in at least some cases some “serial” structures are indeed more similar to each other in derived taxa than in phylogenetically more ancestral ones, as pointed out by authors such as Owen. In this article, we are taking a step back to question whether such assumptions are actually correct at all, in the first place. In particular, we review other cases of so-called “serial homologues” such as insect wings, arthropod walking appendages, Dipteran thoracic bristles, and the vertebrae, ribs, teeth, myomeres, feathers, and hairs of chordate animals. We show that: (a) there are almost never cases of true ancestral similarity; (b) in evolution, such structures—for example, vertebra—and/or their subparts—for example, “transverse processes”—many times display trends toward less similarity while in many others display trends toward more similarity, that is, one cannot say that there is a clear, overall trend to anisomerism.     

The different effects of PEG 6000 and NaCI on leaf development are associated with differential inhibition of root water transport

The inhibitory effects of PEG on whole-plant growth can exceed the effects of other osmolytes such as NaCI, and this has been ascribed to toxic contaminants, or to reduced oxygen availability in PEG solutions. We investigated another possibility, namely that PEG has an additional inhibitory effect on root water transport which in turn affects leaf development. The effects on first-leaf growth of applications of PEG 6000 or isoosmotic NaCI to the roots were determined using hydroponically grown maize (Zea mays L.) seedlings. Leaf growth rates were inhibited within minutes of PEG application to the roots and remained inhibited for days. The inhibitory effects on growth of NaCI, and also of KCl and mannitol, were much smaller. The comparative effects of NaCI and PEG on root water transport were determined by assaying pressurized flow through excised roots. PEG induced a 7-fold greater inhibition of flow through live roots than NaCI. Killing of the roots by heat treatment, to reduce cell membrane resistances to solute penetration, nearly doubled the flow rate for roots in NaCI, but not for roots in PEG. We suggest that the greater viscosity of PEG solutions, as compared with NaCI, may be a primary factor contributing to the additional inhibition of water flow through live and killed roots. PEG did not have additional effects on leaf turgor but had a 3 times greater inhibitory effect than NaCI on the irreversible extensibility of the leaves and induced 16 times more leaf accumulation of the growth inhibitory stress hormone abscisic acid (ABA). We conclude that greater inhibition of root water transport by PEG 6000, as compared with NaCI, leads to additional reductions in extensibility, additional ABA accumulation, and a greater inhibition of leaf growth.     

An automatic device to record the force necessary to compensate plant movements

A device, which automatically and continuously keeps moving plants in fixed positions, has been constructed. The apparatus is based on optical detection of any movements of the plant organ under study. The plant organ is kept in the desired position by means of wires, and the force necessary to apply to the wires to achieve this is recorded. The force reflects any tendency of the plant organ to move. The system is controlled by an Apple II computer. The device can work in one or two dimensions and record compensation forces in a wide range. In the experiments mentioned below, the magnitude of the forces necessary to keep the plants in a fixed position was of the order of 10^-4N.
Circadian leaf movements of Oxalis regnellii Mig. were studied in the device. The leaf rhythm continued, although the leaflets were clamped and the light input on the leaf therefore constant. Circumnutation of hypocotyls of Helianthus annus L. cv. Californicus were drastically reduced in amplitude when the hypocotyls were kept in vertical position by the wires. Since the gravitropic input signal to this system was zeroed by the equipment, the results demonstrated that in the absence of gravitropic inputs the circumnutation reactions drastically diminish. This confirmed that circumnutations of these hypocotyls are influenced by gravity. Finally, the apparatus was used to study phototropic reactions: By clamping phototropically stimulated coleoptiles of Avena sativa L. cv. Seger no gravitational stimulations were involved and the phototropic reaction without interference from gravity could be studied.     

Systematic relationships within the genus Trichomanes sensu lato (Hymenophyllaceae, Filicopsida): cladistic analysis based on anatomical and morphological data

Systematics within the genus Trichomanes sensu lato (Hymenophyllaceae, Filicopsida) have continued to be controversial because of the difficulty in identifying homologies and informative characters within high morphological diversity. Systematic relationships are investigated in this study by using a cladistic approach with 31 anatomical and morphological characters from 20 taxa which correspond to the sections defined by Morton. The results broadly confirm Morton's four subgenera: Trichomanes, Didymoglossum, Pachychaetum and Achomanes. The monophyly of Pachychaetum remains fragile and the section Lacostea appears to be excluded from its traditional subgenus (Achomanes). In addition, by producing two major 'ecological' clades (terrestrial and epiphytic), the analysis shows that several selected characters appear to be strongly linked with the ecology which may have a significant influence on the topology.