ISSN:0044-5983    

DOI:10.1111/j.1438-8677.1989.tb01907.x

年代:1989

数据来源: WILEY

ISSN:0044-5983    

DOI:10.1111/j.1438-8677.1989.tb01908.x

年代:1989

数据来源: WILEY

摘要:

SUMMARYThe ultrastructure of generative cells and vegetative cells ofPapaver dubiumL. pollen was studied with low‐temperature scanning electron microscopy (LTSEM). Liquid propane freezing resulted in a very fast fixation. The on‐line fracturing and observation of the pollen grains was performed at − 130°C. The specimens were fully hydrated and did not undergo morphological changes during the procedure. Unfractured and cross fractured organelles, generative cells, vegetative nuclei and membranes are visible in three‐dimensional images. The LTSEM micrographs show a lobed vegetative nucleus with many nuclear pores and a wrinkled generative cell in longitudinal direction. The vegetative nucleus and the generative cell are situated close to each other but structural connections are not

ISSN:0044-5983    

DOI:10.1111/j.1438-8677.1989.tb01909.x

年代:1989

数据来源: WILEY

摘要:

SUMMARYBegonia salaziensisis the type of sectionMezierea(Gaud.) Warb. The section contains six species, here arranged in three groups, and is considered the most primitive in the genus. Seeds ofB. salaziensiswere obtained from Mauritius and plants grown from these flowered after 16 months. Among all theBegoniaspp. studied hitherto, the placentation ofB. salaziensisappears to be unique. The entire surface of the ovarial cavity is covered by ovuliferous placental tissue. This novelty is placed in an evolutionary transformation series. It is suggested that the placentation inB. salaziensisrepresents the most plesiomorphic state of the character placentation known withinBegoniato date. The etymology of the name Mezierea is given. The accepted species and their synonyms are listed.

ISSN:0044-5983    

DOI:10.1111/j.1438-8677.1989.tb01910.x

年代:1989

数据来源: WILEY

摘要:

SUMMARYA model is developed to calculate the population growth rate of different lineages within a biennial species, in a density‐independent situation. The types differ in the minimum size they require for flowering. The environment is stochastic in the sense that each year a value for combined seed and early seedling survival was drawn from a probability distribution. Other probabilities of survival and growth were entered into the model as fixed rates, which are only dependent on the size of the plant. From demographic data on the delayed biennialsCirsium vulgareandCynoglossum officinale, the model predicts that for both species (1) there exists a unique, optimal value of minimum size or weightw* for flowering, i.e. delayed flowering is profitable, (2) there is weak selection against flowering at sizes larger than optimalw*but strong selection against flowering at sizes smaller than optimalw*, and (3) that optimalw*is 2 g or more. In the field the size requirement was between 1 and 2 g for both species. The optimal value ofw*is mostly determined by the fact that delayed flowering results in an increase in the expected number of seeds and less by delayed flowering resulting in averaging of variable recruitment over the year

ISSN:0044-5983    

DOI:10.1111/j.1438-8677.1989.tb01911.x

年代:1989

数据来源: WILEY

摘要:

SUMMARYThe importance of density‐dependence is addressed by discussing the results of a simple deterministic model for plant populations. The model takes into account that plants, once germinated, do not move. It is shown that density‐dependence can keep sparse populations sparse. Also, it is shown that density‐dependence, even if it affects only a minority of the individuals, can determine the outcome of competition. The effect of density‐dependence on selection for a specific seed size and generation time are di

ISSN:0044-5983    

DOI:10.1111/j.1438-8677.1989.tb01912.x

年代:1989

数据来源: WILEY

摘要:

SUMMARYThe demography ofPlantago lanceolatawas studied in eight grassland habitats ranging from open unmanaged vegetations and pasture on dry sandy soil to hayfields with a high and dense vegetation on loam or clay soils. Data on density and survival of juvenile and adult plants and on aspects of natality were collected. The first two axes of a principal component analysis of these data represented 41% and 25%, respectively, of the total variance. Correlations of the axes with environmental factors of the respective habitats were calculated.The first axis was primarily correlated with light transmission of the vegetation in summer. Thus, survival of juvenile and adult plants during the summer, and in general their density, was highest in open vegetations, while in dense vegetations survival in a severe winter and the number of seeds per plant were higher. Supporting evidence is given for the conclusion that the survival in summer, especially of young and small plants, is positively influenced by this factor. The most important (negative) contribution to the second axis comes from the survival in the second (mild) winter. This axis is significantly correlated with the organic matter and total nitrogen content and moisture content of the soil in winter. This correlation is probably caused by mortality due to water‐saturation of the soil. During the period studied no adverse influence of drought in summer was foun

ISSN:0044-5983    

DOI:10.1111/j.1438-8677.1989.tb01913.x

年代:1989

数据来源: WILEY

ISSN:0044-5983    

DOI:10.1111/j.1438-8677.1989.tb01914.x

年代:1989

数据来源: WILEY

摘要:

SUMMARYAs a result of new collections, the distribution of the rare, pantropical seagrassHalophila decipiensOstenfeld in the Indian Ocean has become much better known. In this paper the species is recorded for the first time on the coasts of Ethiopia, Tanzania and Burma. An updated map of its distribution in the Indian Ocean is presented.

ISSN:0044-5983    

DOI:10.1111/j.1438-8677.1989.tb01915.x

年代:1989

数据来源: WILEY

ISSN:0044-5983    

DOI:10.1111/j.1438-8677.1989.tb01916.x

年代:1989

数据来源: WILEY