In the course of the investigations of the influence of temperature on vital processes it has been observed that at low temperatures the Q10 is about constant, while at higher temperatures it gradually decreases. Several explanations have been offered to explain this decrease. During the last few years two schools draw the attention, the one of Crozier (1924 and following years), the other of Belch radek (1932). Crozier and his collaborators are of opinion that a curve with decreasing Q10 should be regarded as a compound of some parts, each of which should agree fully with the formula of van ’t Hoff—Arrhenius, while the ’’temperature-characteristic” (/«), should have a different value for each part. They demonstrated the possibility of this supposition in a large number of cases, representing usually very complicated processes. Belehradek, however, is of opinion that in all cases where an influence of temperature is studied, a change in viscosity of the protoplasm is the fundamental process. He assumes that the decrease of the Q10 with a rise in temperature will take place gradually only. Wassink (1934) has given a valuable contribution to the solution of this problem, after van den Honert (1930) had pointed out that the temperature-relation can be used for the analysis of limiting processes. Wassink analysed the temperature-curve for the respiration of Phycomyces, where d e Boer (1928) had found a rectilinear relation between temperature and respiration. In this case the Q10 already decreases rather rapidly at lower temperatures. Wassink was able to indicate that this curve is the resultant of two van ’t Hoff-curves, one with a high and the other with a low Qr, (Q5 = 1.60 and 1.10 resp.), older parts of the mycelium producing the former curve, young parts the latter. He could demonstrate that in young cultures the food supply (i.c. glucose) limits the respiration-velocity, an additional limitation by the oxygen supply also being possible; in the older cultures, however, respiration s. str. was limiting. When studying the protoplasmic streaming in the coleoptile of Avena (1934) I observed that the velocity of the streaming movement did not increase above a certain temperature in young objects, while this clearly could be noticed in old plants. A conclusion to parallelity between these observations with those of Wassink was obvious. This was investigated by studying the influence of the temperature on the rate of protoplasmic streaming in coleoptiles of different ages.