A Comparison of the Uptake and Translocation of Some Organic Herbicides and a Systemic Fungicide by Barley: I. ABSORPTION IN EELATION TO PHYSICO-CHEMICAL PROPERTIES

A comparative study has been made of the uptake by and translocationfrom roots of intact barley plants of six herbicides and a systemicfungicide (four triazines, diuron, 2,4-dichloro-phenoxyaceticacid (2,4-D) and ethirimol). Relationships between uptake andtranspiration rate are discussed in the light of the physico-chemicalproperties of these compounds, notably their partition coefficientsin oil/water systems and their dissociation constants. Apartfrom 2,4-D, sorption of these compounds appears to be a passiveprocess. At pH4 the uptake of 2,4-D seems to be influenced bymetabolism; not only may the concentration of this compoundin the transpiration stream be considerably greater than thatin the medium surrounding the roots but absorption by rootsis markedly reduced at low temperatures and by sodium azide. The initial rate of uptake of these compounds correlates reasonablywell with their partition coefficients in olive oil/water orn-dodecane/water systems; likewise the concentration in thetranspiration stream is greater for lipophilic than for lipophobicsubstances. Whereas the hydrogen ion and calcium concentrations of the ambientmedium appear to have no effect on the uptake of compounds withlow pK's, the uptake of those substances which protonate betweenpH4 and pH6 is affected by them. These findings are discussedfrom the viewpoint that the pathways of transport of lipophilicand lipophobic compounds across the roots may differ. Although there is some evidence that retention by roots canlimit transport to shoots, there is no simple inverse correlationbetween the total concentration of the different substancesin the roots and that in the transpiration stream. This questionis discussed in a subsequent paper.     

Comparative anatomy and systematics of Senghas's cushion species of Maxillaria (Orchidaceae)

Using data obtained through anatomy and morphology, we used cladistics to examine the monophyly of Senghas's proposed classification of Maxillaria cushion plants and his placement of Mormolyca ringens. Trignidium obtusum was chosen as the outgroup. Leaves have multicellular hairs sunken in crypts, primarily anomocytic or primarily tetracytic stomatal apparatuses, homogeneous mesophyll, and scattered fibre bundles. Three types of adaxial hypodermis were observed: (1) water-storage cells, (2) fibre bundles scattered among water-storage cells, and (3) fibre bundles scattered among chlorenchymatous cells. Abaxial hypodermis of fibre bundles occurs in several Maxillaria species and in Trigonidium obtusum. At the midvein of the leaf, adaxial mesophyll cells of most species are anticlinally extended and empty, and the abaxial mesophyll is usually collenchymatous. Vascular bundles are collateral and usually in a single series. Pseudobulb epidermal cell walls are thin, or outer walls are thickened. Ground tissue consists of water-storage and assimilatory cells with vascular bundles and associated lacunae scattered throughout. Roots are velamentous and exodermal cell walls are usually n-thickened with tenuous bands of scalarifom thickenings on longitudinal walls. Tilosomes may be plaited, baculate, or spongy. Endodermal cell walls are usually U-thickened and pericycle cell walls are usually O-thickened opposite phloem sectors. Stegmata line the periphery of the thickened pericycle cells opposite phloem sectors in M. picta. Pith may be parenchymatous or sclerenchymatous. According to our phylogenetic analysis, Mormolyca ringens is consistently nested within the cladistic structure of Maxillaria. Therefore, Maxillaria likely is paraphyletic if Mormolyca ringens is recognized as generically distinct. It appears that Senghas's subgroup divisions of the unifoliate pseudobulbous maxillarias may also be artificial.     

Electron Microscope Study of Toxoplasma Cysts in Mouse Brain

SYNOPSIS. Toxoplasma aggregates in sub-acutely and chronically infected mouse brain were studied with special regard to interparasitic relationships, encapsulating wall formation and host-parasite interaction. The individual parasites within a cyst are separated from one another by an opaque substance which also appears as a component of the cyst wall. A second constituent in the wall consists of vesicular and membranous structures which presumably are derived from the endoplasmic reticulum of the host cell. In small cysts, the organising are loosely arranged and maintain the typical crescentic shape whereas, in large cysts, they are tightly packed and polygonal in outline. It is concluded from the data obtained that only the term "cyst" correctly designates these parasitic aggregates.     

Insights into the history of the legume‐betaproteobacterial symbiosis

The interaction between legumes and rhizobia has been well studied in the context of a mutualistic, nitrogen‐fixing symbiosis. The fitness of legumes, including important agricultural crops, is enhanced by the plants’ ability to develop symbiotic associations with certain soil bacteria that fix atmospheric nitrogen into a utilizable form, namely, ammonia, via a chemical reaction that only bacteria and archaea can perform. Of the bacteria, members of the alpha subclass of the protebacteria are the best‐known nitrogen‐fixing symbionts of legumes. Recently, members of the beta subclass of the proteobacteria that induce nitrogen‐fixing nodules on legume roots in a species‐specific manner have been identified. In this issue, Bontemps et al. reveal that not only are these newly identified rhizobia novel in shifting the paradigm of our understanding of legume symbiosis, but also, based on symbiotic gene phylogenies, have a history that is both ancient and stable. Expanding our understanding of novel plant growth promoting rhizobia will be a valuable resource for incorporating alternative strategies of nitrogen fixation for enhancing plant growth.