MICROGAMETOPHYTES IN FLOWERS WITH AND WITHOUT FRUITS OF PHLOX DRUMMONDII (POLEMONIACEAE)

Relationships between microgametophyte numbers per flower and fruit-se t were sought in Phlox drummondii HOOK. This was accomplished by count ing fluorescent-stained pollen tubes in flowers with and without fruit s in 21 populations, and in flowers subject to either supplemental or sparse pollination, and then determining whether the flowers initiate fruits. There was a conspicuous variation in mean pollen tubes per flo wer (3-38) and in percent fruit-set (17%-92%) among populations. Neigh boring populations often differed strikingly in these respects. Flower s with fruits had an average of 15.2 pollen tubes per stigma whereas t hose without had an average of 3.7 tubes. Over all flowers, there was a mean of 3.7 tubes per ovule. There was a strong correlation between mean tube number within populations and their percent fruit-set (r = 0 .86). Supplemental pollination significantly enhanced fruit-set, there by supporting the notion that fruit-set was pollen-limited in most pop ulations. In both natural and artificial pollinations a portion of flo wers without fruits still had several pollen tubes. The occurrence of fruitless flowers containing tubes is explained by attrition factors, like late-acting pollen-pistil incompatibility, and resource limitatio n. Fruit-set could be initiated with one pollen tube, but this rarely occurred in nature. Higher pollen loads were accompanied by higher per cents of fruit-set and number of seeds per flower.