Constancy of the Pentamerous Corolla Phenotype in Natural Populations of Linanthus

The normal number of corolla lobes is five throughout the Polemoniaceae. A survey in 1964 of 34 populations of several closely related species of Linanthus was taken across a transect of Northern California. This survey revealed the presence in every population of only 1% to 4% of flowers deviating from modality. The percentage of variation can therefore be fixed at approximately 2%. The average percentage of deviation to fewer than five corolla lobes was two and a half times larger than the percentage abnormal in excess of five. In the following 2 years, environmental factors were found which disrupted this high percentage of constancy of the pentamerous corolla, thereby causing a substantial increase in the number of abnormal flowers. These factors were the chewing off of the main stem of many plants by ground squirrels, and plants flowering late during the normal season. The increase of abnormals in both cases was hypothesized to be due to the later production of flower primordia when both the length of daylight and the extreme temperatures exceeded the buffered range for some of the plants, thereby more often upsetting the developmental processes. As not all plants showed this breakdown, both genetic and environmental factors are indicated. The ratio of abnormals 5 decreased as the population matured. The conclusion to be reached is that floral structures are normally the most stable of the plant, and yet it is clear that towards the limit of ecological tolerance, conditions may be encountered in which phenotypic variability is enormously enhanced. The evolutionary significance of these findings is briefly discussed.