PYRETHRUM CULTIVATION IN ENGLAND, 1925-37

Small plot and field experiments on the cultivation of the insecticidal pyrethrum plant in England during 1925-37 showed that the plant grew well and produced satisfactory yields of flowers of high pyrethrin content: costings of the field trials, however, showed that the cultivation of the plant was uneconomic. Individual plants grown from seed differed considerably in growth habit and in the pyrethrin content of their flowers.     

Inflorescence Initiation and Leaf Size in Some Gramineae

The morphology of successive leaves on the flowering shoot wasstudied in species of Glyceria, Lolium, and Triticum. The bladesof successive leaves were progressively longer, eventually reachinga maximum, after which the blades of the last few leaves producedbefore heading were shorter. When the longest leaf blade waselongating, dissection of the shoot apices showed that inflorescenceinitiation was taking place. Epidermal cell measurements inTriticum indicate that differences in blade length are due todifferences in the amount of cell extension. It appears that a correlated change occurs in blade morphologyassociated with the onset of the reproductive state of the shootapex, brought about via changes in the amount of cell extension. A study of the effect of different amounts of low-temperatureand different day-lengths on the relation between inflorescenceinitiation and the production of the longest leaf blade showedthat, under some conditions, this relation can be disturbed.     

Cystolith Development and Structure in Pilea cadierei (Urticaceae)

Cystolith formation, structure and composition have been investigatedin leaves and stem internodes of Pilea cadierei (Urticaceae)using a variety of techniques at the light and electron microscopelevels. The development of lithocysts from epidermal cells hasbeen followed. These cells are cytoplasmically similar to otherepidermal cells but possess a much more active Golgi apparatusand more numerous mitochondria. The cystolith is a spindle-shapedbody composed of concentric layers of longitudinally orientatedcellulose microfibrils associated with pectins and other cellwall polysaccharides. At maturity it is heavily impregnatedwith calcium carbonate. Some cystoliths also contain siliconand are covered in a sheath of siliceous material. Cystolithformation occurs at the tip of a peg that grows in from thelithocyst wall. Evidence from ultrastructure suggests that thelithocytst cytoplasm transports carbohydrates to the cystolithvia Golgi vesicles, and organizes the deposition of cystolithcellulose microfibrils via a system of microtubules lying beneaththe plasma membrane that envelopes the growing cystolith. Thepeg is composed of heavily staining amorphous material likethat of an apoplastically sealed cell wall. It is incapableof supporting the migration of lanthanum ions into the cystolith.We conclude that cystoliths are isolated volumes of apoplastthat act as repositories for inorganic salts, principally calciumcarbonate. We propose that calcium ions move into the lithocystprotoplast from surrounding cells and are then transported acrossthe plasma membrane boundary into the cystolith. This proposalconflicts with previous suggestions that calcium enters by diffusionthrough the peg. Cystolith, lithocyst, cell wall, calcium, silicon, cytochemistry, electron probe analysis, Pilea cadierei     

Morphological Modifications of Oocyte Vitelline Envelope from Bufo arenarum during Different Functional States

The submicroscopic morphology of the vitelline envelope of Bufo arenarum's oocyte change significantly during the maturation and fertilization processes. The morphological changes are related to physiological activity in vivo and can be triggered in vitro by experimental procedures. It is our scope to present the ultrastructure differences of fascicular components of the vitelline envelope in coelomic, "pars recta" conditioned, oviductal, oviposited and fertilized oocytes. Our experimental results indicate that artificial "pars recta" treatment of coelomic oocytes arrange the fascicular components as those of oviposited oocyte, although differences still remain indicating that additional maturation processes take place while the egg pass througth the oviduct. Fertilized or activated oocytes which are refractary to sperm penetration, change the vitelline envelope fascicular components orientation giving a submicroscopical image quite different to those of none fertilized oocytes. These ultrastructural changes define in a clear cut manner the functional states of Bufo arenarum's oocyte.     

A bioassay for screening materials influencing feeding in the field slug Deroceras reticulatum (Müller) (Mollusca: Pulmonata)

Test materials were incorporated in wheatflour pellets to measure their effect on slug feeding. The dry weights of individual pellets were measured before and after exposure for 24 h to starved slugs. The method gave consistent results, and can accommodate test materials with different physical properties. The phagostimulant effect of sucrose was confirmed. An extract of the herb tarragon (Artemisia dracunculus) was the most effective antifeedant of the materials used at the concentrations tested. The method is compared with other published techniques.     

Small mammal abundance in relation to microhabitat in a dry sub-humid grassland in Kenya

Six species of rodents and two species of insectivores, live-trapped on a 1 ha grid over a 27 month period, were studied in relation to microhabitat factors in a dry sub-humid grassland in Kenya. Wet season peaks of small mammal species were followed by low numbers trapped during the dry seasons. Species distributions were correlated with different microhabitat parameters, and the separation of species with apparently similar environmental requirements could be explained largely in terms of current knowledge of the biology of the species. Correlation analyses indicated an ecological separation of two closely-related Mus species and of two Crocidura shrews. Overlap indices were extremely high, ranging from 0.556 to 0.877 for all combinations of pairs of species. It seems likely that neither food nor micro-habitat partitioning completely explains the coexistence of the small mammal species and it is suggested that populations are highly transient, moving from refuge areas into temporarily favourable areas during the wet season.     

Age-specific patterns of reproduction in White-tailed and Willow Ptarmigan Lagopus leucurus and L. lagopus

Although many studies report a difference in reproductive success between old and young birds, little is known about how, why and when productivity changes as individuals age. We examined age-dependent reproduction in two bird species that inhabit harsh tundra environments: White-tailed Ptarmigan Lagopus leucurus in alpine areas and Willow Ptarmigan Lagopus lagopus in subarctic Canada. We evaluated reproductive performance in the light of three hypotheses: constraint, restraint and selection. Using cross-sectional and longitudinal data, we observed significant age effects in seven of the eight life history and behavioural traits examined for the two species. However, the pattern of age effects variedconsiderably across life history stages; younger birds generally had smaller clutches, later laying dates and poorer spring body condition, but the nesting success did not vary with age. Brood-rearing and renesting abilities were greater for older parents. The oldest age class of White-tailed Ptarmigan showed reproductive senescence for laying date and clutch size but fledged such a large proportion of the brood that they had the highest overall production of any class. It thus appears that parental experience can compensate for reduced physical ability to produce eggs. Annual mortality rates for breeding females were U-shaped for White-tailed Ptarmigan, with higher rates for young and old birds, but mortality did not change with age in Willow Ptarmigan. Overall, the two species differed in the presence of age dependence for only two traits (renesting ability and annual survival). Age-dependent effects were generally greater for White-tailed Ptarmigan than for Willow Ptarmigan. The patterns of mortality and fecundity we observed in ptarmigan provide general support for the constraint hypothesis of reproductive performance. By examining discrete stages of reproduction, we identified the life history stages where age effects occur and propose proximate mechanisms responsible for these effects.