Rosa setigera is unique among known roses because it is truly dioecious, yet the plants and flowers are almost impossible to sex. Subtle differences in the inflorescences have been detected, for example, there are more flowers per inflorescence on male plants than on female plants and petal expansion from the day of opening to the next day is greater in females than in males; in addition, pollen from female plants appears somewhat collapsed and does not germinate. Pollinators (Apis mellifra, other bees (Apidae), Eristalis tenax and other Syrphidae: Diptera) visit the blossoms mostly in the morning to collect pollen (there is no nectar) and in choice experiments do not discriminate between male flowers and female flowers on landing, but do spend significantly more time on the former. They d o discriminate between freshly opened flowers and day-old flowers, and show marked preference for bouquets of five flowers us. single flowers. The female plants, with smaller inflorescences and lesser interest to pollen foraging insects, seem to encourage them to forage at more flowers than do their male counterparts. This may be biologically significant for effective pollen flow in a dioecious pollenonly plant with pollenivorous pollinators.KEY WORDS:-Pollinators -pollinator behaviour -insect foraging -floral size -floral ageing inflorescences -Apis mellijka -Eristalis lenax -Apidae -Syrphidae. 229 0024-4066/90/070229 + 15 S03.00jO

In Canada, Ptelea trifoliata L. (Rutaceae) is restricted in its natural distribution primarily to the Lake Erie shoreline. Although it is locally successful as a colonizing species, it appears inhibited from either exploiting a large area of shoreline habitat or extending its range inland. The morphological descriptions of the hop tree's sexual expression have been ambiguous; however, our observations show this species to be clearly dioecious, with only about 2% of the otherwise male plants producing a few hermaphroditic flowers, and fruit. The sex ratio is strongly skewed toward the males, which produce more and showier inflorescences than female plants. Despite this, the insect visitation rates are not significantly different between the sexes. For pollination, the hop tree is a polyphilic generalist, dependent on a diverse array of insects, including short-tongued bees, wasps, and flies. It is totally dependent on wind-dispersed seeds for establishment, lacking the ability to grow clonally. In a dioecious species of disturbed habitats, this is exceptional.

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1976. Evidence of microevolution in an escaped pear population. Can. J. Bot. 54: 2857-2867. A population of 87 wild pears (Pynis commrrr~is L.) of fruiting age, growing in a relatively homogeneous old field environment, was found to be highly variable. with coefficients of variation up to 40%. Generally, variability was greater in size than in meristic characters, in meristic characters than in shape characters, and floral traits were less variable than vegetative. Tree trunk core samples showed the population to have been founded circa 1905 to 1908, by three trees. Establishment ofprogeny began about 19 years later. The population is high in 1975 and still expanding logarithmically. Numerical taxonomic clustering techniques using morphological characters showed the population to contain two very diverse groups. Trees of both groups are of all age classes and are distributed randomly over the site. Trees phenetically like and unlike the oldest trees were analysed as to growth rates. Those trees similar to the oldest trees were significantly slower in growth rate than trees that were morphologically dissimilar. This suggests that natural selection is operating to remove the less fit genotype from the population.

Round-leaved greenbrier, Smilax rotundifolia L. (Liliaceae), is an insect-pollinated, dioecious, understorey vine. Its pollen is linked by viscin threads, effectively preventing wind dispersal. In most sites populations are not sexually reproductive, being unisexual, but do not spread vegetatively. Where both sexes occur, fruits and seeds are produced naturally. Artificial cross-pollinations can enhance the level of fruit and seed set. Understanding the biology of this threatened species is important for habitat management and conservation. Key words: dioecy, greenbrier, viscin, floral form, conservation, pollination.