Experimental work was started as an attempt to obtain further evidence of the estival reduction of size observed in diverse freshwater organisms, and to pave the path for a further analysis of the biological mechanisms involved. This paper is an advanced report, deprived of the numerical data in extenso, that will be included in a more extensive communication on the action of temperature on organic morphogenesis, to be published later. Standard unialgal cultures of a race of Scenedesmus obliquus (Turp.) Kuetz. were run in vessels with 300 mI. of Detmer medium plus soil extract, the pH was maintained near 7--7,2. After 12--15 days in a thermostat, with continuous illumination (electric bulb of 40 watts at 10 cm), density of cultures ranged from 4.000 to 20.000 cells per cubic millimeter. In total 45 populations grown under temperatures ranging from 13 to 29 ° C have been studied and 50-- 70 cells of each sample measured. Chemical analysis of ceils were carried on 200 ml. of culture

12 pages, 2 figures, 9 tables

Peer reviewed