thrum

serpyllifolia, 216 intermorph crosses, 153 intramorph crosses and 154 unpollinated controls were distributed among flowers on 129 small plants (71 pins and 58 thrums) grown from seeds harvested from the population at Whiteside Mountain.

purpurea thrums as the paternal parent was collected from unbagged plants.

- Haustonia serpyllifolia expresses the dimorphism of pollen size often found in distylous plants: thrum pollen is significantly larger (F = 826.3, P [less than] 0.0001), although the distributions of pollen sizes produced by pins and thrums overlap (pin pollen diameter = 7.7 [+ or -] 0.4 [[micro]meter] (mean [+ or -] SD), thrum pollen diameter = 9.2 [+ or -] 0.7 [[micro]meter]).

For Houstonia serpyllifolia, pins set more fruits and more seeds per fruit than thrums following intramorph crosses, suggesting that pins might be slightly more intramorph-fertile (Table 1).

Pins produced a higher percentage of large but seedless capsules, whereas thrums had more capsules that were only slightly swollen.

Unpollinated control flowers of Houstonia serpyllifolia produced an unexpectedly high percentage of fruits, with pins showing 14% fruit-set and thrums showing 7%.

For Houstonia serpyllifolia, pins set more fruits and more seeds per fruit than thrums following intramorph crosses or no pollination, suggesting that pins might be slightly more intramorph- or self-fertile.

Thrips probably transferred small amounts of compatible pollen by moving between nearby flowers, particularly when pins and thrums were grown in close proximity.

The flat that flowered contained only thrums, so data on pins are lacking.

Performing this operation to get a rough correction for thrip effects, we get the following levels of intramorph fruit-set: for Houstonia serpyllifolia: pins 13%, thrums 8%; and for H.