摘要:

Summary1. The process of oosorption in insects is discussed with regard to the chronology of events which occur during resorption of oocytes and the role of their auxiliary cells.2. A theory of neuroendocrine control of oosorption is put forward, suggesting that cessation of juvenile hormone secretion is the most important factor leading to this degradative process in spite of the fact that oosorption and vitellogenesis can occur simultaneously.3. The diversity of behavioural, ecological and physiological factors which promote oosorption is discussed with an emphasis on differences and similarities among representatives of major insect groups, and the manner in which oosorption provides an ovipositional strategy.

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1975.tb01058.x

出版商:Blackwell Publishing Ltd    

年代:1975

数据来源: WILEY

摘要:

Summary1. Whatever heirarchical system of classifying graptolites is adopted, perhaps even raising them to the rank of phylum, the hemichordates includingRhabdopleuraandCephalodiscusremain their closest relatives.2. Benthonic graptolites preceded and outlasted the planktonic graptolites which exhibited spectacular morphological changes related to a change to a holoplanktonic mode of life and a probable hermaphroditism.3. The resorption of the narrow‐end of the larval skeleton (prosicular cauda) enabled the construction by an outer layer of secreting tissue (extrathecal tissue) of a hollow nema, the dark hollow rod which supported gas‐filled tissue and which in turn conferred buoyancy on the colony.4. The nema provided access to the exterior of the colony for extrathecal tissue which began to strengthen the outside of the prosicular and metasicular parts of the larval skeleton with laminated cortical deposits before the first non‐sicular individual of the colony was budded.5. Attachment of the larval stage in those graptolites having a basal disc or ‘roots’ was by the first‐formed extrathecal tissue through a resorbed cauda.6. Initial attachment of the larval stage in certain encrusting graptolites such asIdiotubusmay have been by a larval individual lacking any scleroprotein periderm as a skeletal sheath.7. After initial attachment the extrathecal tissue continued to secrete additional layers of cortical tissue mostly for the purposes of strengthening the colony.8. Feeding in graptolites was by ciliated lophophore often positioned so as to take maximum advantage of currents flowing from the dorsal to the ventral side of the colony, for example inDictyonemaandMonograptus.9. The main function of the nema, particularly of those nemata bearing vanes, was as a support for vacuolated tissue which imparted a holoplanktonic mode of life to planktonic (sensu Zato) graptolites.10. Attachment of colonies (rhabdosomes) as groups of colonies (synrhabdosomes) was by the extrathecal tissue issuing from the tips of the nemata: such associations were probably sexual rather than for reasons of buoyancy.11. Attachment of planktonic graptolites to floating algal fronds is an unnecessary hypothesis. Although an undoubted occurrence of graptolites in rocks containing large quantities of carbonaceous matter is beyond dispute, the plant or animal nature of this material has never been established. A symbiotic relationship of planktonic graptolites with a marine, formless alga, perhaps involving the extrathecal tissue, remains a possibility.12. Although different species may have lived at different depths, the full vertical range of the holoplanktonic graptolites was probably small. The evidence advanced for depth zonation is considered inadequate.13. Planktonic graptolites were essentially tropical to temperate in distribution with the bulk of the species and individuals in the former environment.14. Automobility of graptolite rhabdosomes was an unlikely mechanism and does not readily account for the morphology and distribution of the graptolites.15. Most planktonic graptolites were suspended beneath a nema coated in extrathecal, vacuolated (gas‐filled) tissue: changes of position in the water were by passive response to ocean currents.16. The nature of the graptolite zooid is considered unsolved, although it may have been essentially like that of the extantRhabdopleurain having paired lophophores.17. It is possible that a modified pre‐oral lobe was capable of secreting both fusellar (inner) and cortical (outer) layers of the periderm.18. The extrathecal tissue itself would in that case have been derived both from the nemal tube and the the

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1975.tb01059.x

出版商:Blackwell Publishing Ltd    

年代:1975

数据来源: WILEY

摘要:

Summary1. Peculiarities in the ecology of planktonic blue‐green algae are reviewed in relation to recent advances in understanding their physiological characteristics.2. Dense water‐blooms are always the result of buoyant migration of existing populations to the lake surface under calm weather conditions. The size of the population is the direct result of photoautotrophic growth, and is dependent upon light and the availability of inorganic nutrients; it is apparently enhanced by moderately high water temperatures, high pH, low oxygen tensions and possibly, the presence of organic solutes. The relative effectiveness of these factors is untested.3. Buoyancy is imparted by gas vacuoles whose principal function is to regulate the position of the alga in the water column. Control is effected by two mechanisms: (i) ‘dilution’ of newly produced vacuoles during active cell division; (ii) changes in cell turgor‐pressure acting on the gas‐vacuole structure. Gas‐vacuole production is greatest at low light intensities and the alga becomes more buoyant; at higher light intensities, increased turgor‐pressure collapses the weaker vacuoles causing the alga to lose buoyancy.4. Potentially, algae are able to poise themselves at an optimum point in the light gradient, usually towards the bottom of the euphotic zone, where the algae are likely to encounter the conditions most favouring their growth.5. Different species of blue‐green algae differ in the typical sizes of their colonies and, hence, in their rates of controlled movement. These differences are interpreted as hydrodynamic adaptations to the variations in turbulent water movements to which the algae are subject.6. Populations of single‐filamentousOscillatoria agardhiiandO. rubescenscome to occupy the stable metalimnia of stratified lakes, provided that they are located within the euphotic zone.7. The large stream‐lined colonial forms occur mainly in polymictic lakes and in the unstable epilimnia of stratified lakes where light penetration is restricted to the superficial layers. These algae are adapted to sink or float rapidly to the optimum depth when turbulence subsides. Because of their potentially high rates of movement, it is the large colonial forms that commonly form blooms.8. Bloom formation can occur when most of the algae possess excess buoyancy. Excess buoyancy is acquired when the photosynthetic rate is insufficient to develop the necessary turgor‐pressure to cause collapse of the vacuoles. Photosynthesis may be sufficiently impaired under four circumstances: (i) during turbulent circulation of the population over a depth that significantly exceeds the euphotic depth; (ii) in the absence of light (e.g. at night): (iii) at limiting concentrations of carbon dioxide: and (iv) when the algal population is senescent.9. Because bloom‐formation depends upon the coincidence of persistent algal overbuoyancy with calm weather, its occurrence is incidental, and serves no vital function in the biology of blue‐green algae.10. Some possible causes for the occurrence of blue‐green algal blooms in a relatively restricted range of water bodies are discussed. Large bloom‐forming populations are probably restricted to moderately rich, mildly alkaline, thermally unstable lakes in all regions, except those which are permanently cold. Extremes of poverty or richness of nutrients, short water‐retention times and low pH seem to be factors which select ag

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1975.tb01060.x

出版商:Blackwell Publishing Ltd    

年代:1975

数据来源: WILEY