Factors Controlling the Distribution of Tilia Cordata at the Northern Limits of Its Geographical Range Iii. Nature and Causes of Seed Sterility

SUMMARY Failure of regeneration of Tilia cordata in north‐west England is associated with almost complete sterility of the seeds. Between 1964 and 1979 significant numbers of fertile seeds were recorded only after the exceptionally warm summer of 1976. In contrast, large or moderate crops of fertile seed were produced at sites in central and southern England in at least 8 of these years. Comparative studies of pollination, fertilization and seed development at sites in north‐west England, central England and northern France show relatively small differences in the proportions of flowers which are pollinated but significant differences in the rates of extension of pollen‐tubes. In samples collected in 1977 from north‐west England germinated pollen was present on most receptive stigmas but many pollen‐tubes extended only a short distance down the style and the few which reached the base appeared to be arrested in the ovary wall. In French samples pollen had germinated on all receptive stigmas and numerous pollen‐tubes had reached the ovary within 2 days of pollination. These differences cannot be attributed to variation in the incidence of self‐pollination, as at least some trees of T. cordata in northern populations are almost certainly self‐fertile. The differences are correlated with temperatures at the time when the samples were collected. When pollen‐tubes are growing in sucrose solutions, their rate of extension shows a large response to temperature over the same range (15 to 25°C). Analysis of records shows that temperatures in north‐west England at the time of flowering are frequently too low to permit fertilization. This is confirmed by studies of the development of ovules. Although a high proportion of ovules in T. cordata lack embryo sacs, normally at least one is functional in each ovary. In over 90% of fruit formed in north‐west England, embryos are contained in none of the 10 ovules. In less than 10% of fruit partly developed embryos are present, suggesting that temperatures are also too low in late summer to allow completion of their development.