Early and late post-tetanic potentiation,and post-tetanic block in a monosynaptic reflex pathway

Observations have been made upon the nature of early and late post-tetanic potentiation and upon post-tetanic block of presynaptic collaterals with particular reference to behavior in circumstances of varied duration and frequency of conditioning stimulation. Early potentiation is most conspicuous following brief tetani at high frequency, late potentiation following long tetani, much lower frequencies being all that are needed. The two phenomena thus are distinguishable and separate. Dorsal root electrotonus produced by stimulations of varying duration and frequency is described, and the similarity between behavior of the D.R.IV R. electronic potential and early potentiation demonstrated. It is shown how early potentiation and post-tetanic block are due to the same process (hyperpolarization) at different intensities. The view that the agency for potentiation is associated with augmented presynaptic action due to hyperpolarization is confirmed. A diagram is constructed to indicate the probable temporal courses of early and late potentiation.     

Effects of food variability on growth rates, fledging sizes and reproductive success in the Yellow-eyed Penguin Megadyptes antipodes

Effects of a change of diet on growth rates and fledging sizes of Yellow-eyed Penguins Megadyptes antipodes were examined at two breeding areas on South Island, New Zealand, during two breeding seasons. An adverse change in diet was observed in the second season. Evidence for this included depressed growth rates of weight, differential growth of weight and most morphometric parameters between one- and two-chick nests in the second season, lower fledging weights, lower adult body weights, delayed moult, higher chick mortality and higher adult mortality during moult. The change in diet is suggested as being from one including oil-rich prey species, to one of oil-poor species.
Growth rates of first- and second-hatched chicks, and of survivors and non-survivors within a brood were not significantly different in either season, and growth rates of two-chick broods were only slightly slower than one-chick broods for some parameters in the second season. This, and synchronous hatching of chicks, equal egg-size and lack of sibling competition during feeding sessions, suggests that brood reduction is not an option available to Yellow-eyed Penguins, and that food supply may not be a limiting factor in the majority of breeding seasons.
Few changes in growth rates of morphometric parameters at either breeding area, and similar absolute sizes at fledging, indicate that slowing of growth rates of morphometric parameters only occurs when feeding conditions are so bad as to result in mortality and that, although fledging periods may be longer, patterns of development remain essentially unchanged.     

Variation strategies of plants in heterogeneous environments

Plant structures exhibit five principal classes of variation in heterogeneous environments, namely uniformity, continuous lability, genetic specialization (polymorphism), environmentally- or statuscued alternatives (conditional choices) and multiple strategies—the simultaneous operation by one plant of distinct types of structures that perform the same function. Multiple strategies are a diverse but neglected class that includes simultaneous cosexuality (hermaphroditism and other monomorphic sex conditions), facultative cleistogamy, heteromorphic diaspores, and reproduction by both seeds and ramets. An analysis of seven functions in the angiosperm flora of New Zealand shows that uniform and labile strategies considered jointly are most common, and multiple strategies are more common than either polymorphisms or conditional choices. Phenotypic models of the natural selection of structural variation are presented. They predict the general conditions under which multiple, conditional and uniform strategies are selected when the environment is spatially heterogeneous for either parents or their offspring. The models can explain many features of variation strategies, including why multiple strategies are a plant speciality, why conditional strategies such as sex choosing are rare and random choices are even more rare (unknown?), and why some self-fertilizing plants have distinct cleistogamous flowers. The models also suggest further avenues of research.     

Spore germination and isozyme patterns in the apomictic fern Cyrtotnium falcatum

Progeny from a plant of the apomictic species Cyrtomium falcatum were examined by protein electrophoresis. These progeny were produced from spores originating in sporangia with differing spore numbers. Analysis of zymograms indicates that variant banding patterns were exhibited in 16.1% of the progeny sampled. These anomalous phenotypes suggest that some apomictically reproducing plants can produce genetically variable progeny.     

THE OCCURRENCE AND FUNCTIONS OF ULTRADIAN RHYTHMS

Ultradian oscillations with periods between 5 min and 4 h have been described in cell-free extracts, single-celled eukaryotes, cultured cells and embryos. Whereas some of these potentially oscillatory systems (e.g. glycolysis) may only exhibit this type of behaviour rarely if at all in vivo , other ultradian oscillators in lower eukaryotes are rhythms and probably have timekeeping functions. Rhythms with ultradian periods of 10 min to 20 h in oxygen consumption and carbon dioxide production have also been studied in endotherm animals: these rhythms may be modified by variations of environmental parameters and by circadian and infradian synchronizers. Interspecies and interstrain differences strongly suggest that these rhythms are endogenous and have a genetic origin. We suggest that the temporal organization of biochemical and physiological processes facilitates optimization of thermodynamic maintenance of the organism within the random fluctuations of its physicochemical environment and contributes to genetic selection.     

Chemistry and vegetation of highly acidic streams

An account is given of the water chemistry and photosynthetic flora of waters in England with a pH value of 3.0 or less. Of the fifteen sites found, thirteen were associated with coal-mining, one with a barytes mine and one was an industrial effluent. Flowing waters predominated, but in some cases pools in direct contact with the streams were also present. All waters carried high levels of heavy metals and silicate, and most carried moderately high levels of phosphate and combined inorganic nitrogen. The total flora consisted of twenty-four algae, two mosses and two flowering plants. Of these, three algae and one flowering plant were found in pools only. Euglena mutabilis was both the most widespread species, and often also the most abundant. The number of species present in a reach showed a positive correlation with pH during late summer, and a negative correlation with total acidity in winter. Only four of the algal species found in England are certainly included in the U.S.A. literature. Some, but not all, the apparent differences between the two floras are very probably due simply to taxonomic problems.