The publication of Blackman's *) ..Optima and Limiting Factors” was the cause of many new investigations to be made of the relation between temperature and the rate of physiological processes. Whereas before 1905 the experimentators beleived the optimumcurve to be the right form to express the connection between temperature and reaction-velocity Blackman lays stress on a new point of view and comes to the conclusion that a timefactor, until then neglected, comes into play: if it were possible to make an observation at different temperatures during a time zero, there would appear to be no optimum at all and the curve, calculated at infra-optimal temperatures, would gradually mount without any falling off taking place, so that van 't Hoff’s rule on reaction-velocity of chemical processes which increases two—to threefold for every 10° C. rise of temperature might also be applied in biological processes. The optimum hitherto found is to be regarded as a point of the curve changing its place with the time of experiment. Miss Matthaei’s experiments on the influence of temperature on assimilation led him to this theory.