SUMMARYThe fluoreseein test was found to be a rapid and reliable method of assessing pollen viability in onions. This test revealed the presenee of many pollen grains which contained cytoplasm but which were inviable as judged by germination on artificial media. There was very great variation in pollen viability between flowers on a plant, between anthers on a flower and between different parts of a single anther. Both viable and inviable grains oecurred in large clumps consisting chiefly of one type of grain. Plants flowering in August whieh were nominally male fertile and had pollen grains with cytoplasmic contents, nevertheless, showed very high levels of pollen inviability. Possible reasons for this are considered.

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The rapid extension of the world's arid areas – some five million hectares annually – has greatly stimulated interest in the development of agricultural systems sustainable in areas of extremely low rainfall. The possibilities naturally depend very much on local circumstances, but investigation of Prosopis and Acacia species under the severe conditions of the Wahiba Sands of Oman indicates that they have considerable potential under extremely arid conditions.

INDEX WORDSAllium cepa, onion, pollen viability SUMMARY The pollen viability of onions in a glasshouse was recorded from May to October 1975, using the fluorescein test. The average viability was 60-95 % for most of this period but fell to less than 1% during the last two weeks of August. There was great variation in pollen viability between anthers within a flower and between flowers within a head. Attempts to induce pollen inviability by low temperature treatments at various stages of inflorescence development were unsuccessful. Low levels of pollen inviability appear to be a characteristic feature of onions, but the high level ofinviability which was found both in this and in a previous season was associated particularly with the August period.

SUMMARYIncompatible pollen tubes grew at approximately the same rate as compatible pollen tubes in the styles of Brassica oleracea. This is further evidence that in this species the chief barrier to the growth of incompatible pollen tubes is the stigmatic surface and that there is no positive inhibition of incompatible pollen tubes in the style. There were usually very many more pollen tubes at the top of the style than at its base, which indicated considerable variation in growth rate between the pollen tubes in a single style. The fastest pollen tubes in a style typically had a growth rate of 0.40-0.70 mm/h but the mean rate of growth was 0.14-0.34 mm/h.