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21.
Dissolved inorganic 14carbon, fed to a small portion of thetissue of Conocephalun conicum, travelled along the tissuesfrom cell to cell at rates of about 2.0 to 1.7 µm 5–1,which is of the same order of magnitude as streaming rates inthese cells. Trans-cellular transport can take place againsta pressure gradient, but pressure gradients of 200 kPa or morein either direction reduced the trans-cellular transport significantlywithout reducing streaming rate and are, therefore, thoughtto have induced ‘valving’ effects at the plasmodesmata.Ammonium ion reduced or inhibited transport without reducingstreaming rates and is, therefore, thought to have inhibitedan active component in the plasmodesmata. Thus the results obtainedfrom tandem cells of Nitella can be transferred to tissues. Key words: Conocephalum, transport, plasmodesmata  相似文献   
22.
Protein bodies isolated from the starchy endosperm of ungerminatedsorghum exhibited some autolytic activity but seemed incapableof significant self-hydrolysis. Enzyme assay, transmission electronmicroscopy, sodium dodecyl sulphate—polyacrylamide gelelectrophoresis and amino acid analysis revealed that a proteinaseextract from germinated sorghum could degrade the protein bodiesin a manner resembling that which takes place in vivo. The proteinbodies were degraded mainly from the periphery. Glutelin (matrixprotein) was first hydrolysed, followed by the prolamin proteinbody protein. Proteinase extracts from both the germ and endospermof germinated sorghum were capable of degrading the proteinbodies. This finding is consistent with the concept that theproteinase is synthesized in the germ and then secreted intothe starchy endosperm during germination. Key words: Sorghum bicolor, protein body degradation, proteinase.  相似文献   
23.
Griffis, K. & Chapman, D. J. 1990 10 15: Modeling Cretaceous-Tertiary boundary events with extant photosynthetic plankton: effects of impact-related acid rain. Lethaia , Vol. 23, pp. 379–383. OSIO. ISSN 0024–1164.
An acid rain phenomenon has previously been proposed as one of the consequences of a bolide impact contributing to the extinctions at the Cretaceous-Tertiary boundary. This hypothesis has been tested by observing the growth responses of four organisms under simulated acid rain conditions. Two of these phytoplankton, Ditylum (a diatom) and Thoracosphaera (a dinoflagellate). are genera that persisted through the boundary, while the other two, Coccolithus (a coccolithophorid) and Gonyaulax (a dinoflagellate), are post-boundary genera. Ditylum and Coccolithus survive the acid rain simulation. but with the loss of scales in Coccolithus . The two dinoflagellate are sensitive to acid rain simulations. with Gonyaulax unable to survive beyond seven days. The results indicate that acid rain may have contributed to the Cretaceous-Tertiary boundary extinctions. but that the changes resulting from the acid rain were not as severe as postulated or were short-lived and quickly dissipated. ▭ Acid rain, coccolithophorids. Cretaceous/Tertiary extinctions, diatons, dinoflagellates, phytoplankton .  相似文献   
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The fossil record of the Pomacentridae has been based on species from six genera. The status of these taxa is reviewed. Only two are tentatively accepted as belonging to the Pomacentridae: Chromis savomini from the Miocene of Algeria and Izuus nakamurai from the Miocene of Japan. In addition, a new genus and species Palaeopomacentrus orphae is described from the lower Middle Eocene of Monte Bolca, Northern Italy. This is the first substantiated record of a pomacentrid from the Eocene, and represents the oldest record of the family.  相似文献   
26.
Most temperate butterflies exhibit a tightly synchronized unimodal adult emergence to facilitate mate location. Exceptions are presumably subject to unusual selection pressure. This study examines the pattern of emergence in Maniola jurtina , which was found to exhibit both unimodal and bimodal emergence patterns at different sites in south-east England. The bimodal pattern was found on chalk grassland; elsewhere the emergence was unimodal. Adults from each emergence peak rarely meet, suggesting that there may be some degree of reproductive isolation. Morphological measurements and electrophoretic analysis of allozyme frequencies are carried out to quantify differentiation between emergence peaks. Captive stock was reared to examine differences in the immature stages. Butterflies from each emergence differ significantly in most morphological variables measured, those from the second peak tending to be smaller. The immature stages differ in morphology and longevity of the egg stage. Allozyme frequencies did not differ between peaks, suggesting that they are not reproductively isolated. Explanations for the maintenance of differences between emergence peaks despite gene flow are discussed. I propose that division of offspring between two emergence times may have evolved to avoid the risk inherent in placing all offspring in one peak which may be rendered inviable by temporal fluctuations in habitat quality.  相似文献   
27.
Function and biological role of morphological specialization in desmognathine salamanders are analysed in the light of studies of feeding in Leurognatthus marmoratus. Nine morphological features uniquely characterize the Desmognathinae as compared to its sister group, the Plethodontinae, and other salamanders: (1) heavily ossified and strongly articulated skull and mandible; (2) flat, wedgelike head profile; (3) stalked occipital condyles; (4) modified atlas; (5) modified anterior trunk vertebrae; (6) atlanto-mandibular ligaments; (7) enlarged dorsal spinal muscles; (8) enlarged quadrato-pectoralis muscles; and (9) hind limbs relatively larger than forelimbs. Dorsoventral head mobility is increased at the atlanto-occipital joint by the stalked occipital condyles which simultaneously increase the mechanical advantage of the hypertrophied axial muscles that cross the joint. During head depression the atlanto-mandibular ligaments are placed in tension. Force generated by the quadrato-pectoralis muscles is transmitted directly to the mandible, creating a powerful bite with the jaws in full occlusion. Desmognathines use an efficient static pressure system for subduing and/or killing prey items held in the jaws, not a kinetic-inertial mechanism, as previously suggested. Leurognathus exhibits a behaviour ('head-tucking') unique to desmognathines that is consistent with the static-pressure hypothesis. Several desmognathine features (1, 2, 5, 7, 9) are not explicable as adaptations for feeding; these function as locomotory specializations for burrowing, especially for wedging under rocks within and alongside streams. Desmognathines use head-tucking during such wedging and burrowing movements, thus locomotory specializations act in concert with the feeding specializations. We suggest that origin of the atlanto-mandibular ligaments can be considered a 'key innovation' in that it allowed the secondary invasion of stream habitats by adults of ancestral desmognathines.  相似文献   
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Maternal Effects on Offspring Quality in Poeciliid Fishes   总被引:2,自引:0,他引:2  
SYNOPSIS. We evaluated the effects of maternal environment onoffspring size and composition in three species of poeciliidfishes. We chose food availability as the environmental factorfor study. Mature females were assigned to either high or lowfood for an interval of time, then randomly reassigned to highor low food, with the restriction that there be equal numbersin each of four treatments: high-high, high-low, lowhigh, andlow-low food availability. The three species chosen for studydiffer in the pattern of maternal provisioning. Poecilia reticulataand Priapichthys festae mothers provide all resources necessaryfor development as yolk, prior to fertilization. In contrast,Heterandria formosa mothers continue to provision the youngthroughout development. These species also differ in whetheror not they have superfetation, or the ability to carry multiplebroods of young in different stages of development. P. reticulatadoes not have superfetation while the other two species do.We were interested in whether the pattern of maternal provisioningor superfetation influenced the maternal effect. The two lecithotrophicspecies responded to low food by producing larger young withgreater fat reserves. H. formosa, the matrotrophic species,responded to low food by producing smaller young. We proposethat the production of large young in the face of low food availabilitymight represent adaptive plasticity; matrotrophy might representa constraint that prevents such an adaptive response. Superfetationhad no impact on this maternal effect.  相似文献   
30.
1. ‘Placer’ mining for alluvial deposits of gold in a number of stream systems in interior Alaska represents a major disturbance to the stream bed and affects habitat for biotic communities. 2. The potential of analysing aerial photographs to map changes in channel habitat and morphology within gravel-bed rivers is outlined with reference to the impact and recovery of Faith Creek, a second-order stream with a history of placer mining. 3. A strong correlation between the reflectance of the channel bed and water depth is necessary to use the technique succesfully, together with a knowledge of the effects of ‘broken’water on the spectral characteristics of rivers. 4. Image analysis demonstrated that a wide range of water depths and instream mesoscale habitats existed prior to mining. During mining, the stream was confined to a channellized reach with negligible deep water or habitat diversity. 5. Since mining ceased the stream has abandoned its channellized course and formed a new channel with few deep pools. It is suggested that geomorphological recovery and associated habitat recovery takes a number of large flood events and is likely to require more than 10 years.  相似文献   
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