AN INDIVIDUALS STATUS IN POPULATIONS OF ISOETID SPECIES

Individual size and fecundity of Isoetes lacustris L., Lobelia dortman na L. and Littorella uniflora (L.) Aschers., were studied in relation to the depth gradient of an oligotrophic lake. The plants examined inc luded individuals growing in the central part of the population habita t and in the deep and shallow border zones. Zone-related variation in population habitat and structure over the depth gradient has been demo nstrated. The Isoetes population is structurally divided into central and shallow-water zones; the Lobelia population into central and deep zones; the Littorella population into shallow and wave-disturbed, as w ell as into deep border zones. Isoetes individuals in the central zone of the habitat have a rosette size of 87.1 +/- 37.8 cm2, root length of 10.9 +/- 3.6 m and 839 +/- 422.6 macrospores. Corresponding figures in the shallow, non-disturbed zone are 116.8 +/- 47.1 cm2, 17.5 +/- 6 .6 m and 2289.3 +/- 1167. Lobelia individuals in the central zone of t he habitat have a rosette size of 10.6 +/- 4.1 cm2, root length of 5 /- 1.9 m and 1065.3 +/- 24.6 seeds. Corresponding figures in the deepe st zone are 17.2 +/- 6.8 cm2, 5 +/- 2.1 m and 1213.5 +/- 245.6. A smal l group of large and fecund individuals whose growth cycle has taken p lace in the border zones play a significant role in the population reg eneration process: Lobelia in the deep zone and Isoetes in the shallow area not disturbed by waves. In every population (and habitat) zone, fecundity is a function of individual size, and variation in size is a lways greater than variation in fecundity. There are more small indivi duals in a population than indicated by the normal distribution (Gauss curve). Distributions of individual size- and fecundity-defining char acters are always positively skewed.