Biosystematic studies of Spergularia media and S. marina in the Netherlands I. The morphological variability of S. media

The present paper, the first of a series of four on the biosystematics of the Dutch halophilous species of Spergularia, reports the results of a study of the morphological variability of S. media by means of a number of population samples from all parts of the Dutch area, supplemented by the rearing of plants from seed samples in the experimental garden. The seeds of S. media are usually winged, but in the area of the Dutch North Sea Islands, sometimes populations are encountered containing up to 70% plants with exalate to subalate seeds. The development of the seed wing is chiefly determined by genetic factors, but to some extent modifiable by different environmental conditions. The seed wing is too variable to be of decisive diagnostic value to distinguish S. media from S. marina. Plants with exalate to subalate seeds produce seedlings that are significantly smaller than the seedlings of plants with broadly winged seeds. Fruit- and calyx lengths vary too widely to be of diagnostic value either. The number of stamens plus staminodes is usually 10, rarely 9; the number of fertile stamens is 8 to 10 in nearly all flowers of a population, but may be as low as 0 to 3 per flower in some individual plants. The number of fertile stamens is primarily determined by genetic factors. The growth habit and the vegetative characters appear to exhibit only phenotypic variation.