The temperature is a very important factor in the development of the growingpoint in the iris bulb. Many publications deal with these experiments, but little research on the relationship between the metabolism of the bulb and temperature has been done. For that reason the relation between the respiratory activity and the temperature factor has been studied closely. The carbon dioxide output and the oxygen uptake of the bulbs have been measured by self-registering gas analysers. The environmental circumstances during the measurements were in most cases similar to normal conditions of storage. After lifting, the respiration rate of bulbs decreased sharply at high temperatures such as 25° C and 30° C. After a relatively short period of about 3 weeks a steady state respiration is reached at a low level of activity. Only very slowly does the respiration rate increase during the rest of a year’s storage period at 25° C or 30° C. The ‘van Vliet’ and ‘Wedgwood’ bulbs can be stored at these temperatures for a year without losing their viability. The development of the central growingpoint in the bulb is highly retarded under these circumstances, and these retarded bulbs are particularly suitable for physiological research. During a period of 2 or 3 weeks the respiration rate of the bulbs remains in a steady state at a temperature of 25° G (storage condition). If the bulbs — coming from the storage conditions — are transferred to lower temperatures, an increase of the respiration rate is observed within a day. At 15° the increase of the activity is greatest and after a few days a new steady state is reached. This effect is called the 15° effect. There is a relation between the increase of the respiratory activity and an incipient growth of the central growing-point. The optimum temperature of the incipient growth likewise lies at a temperature of 15°. Both the developed respiration and the growth activity can be reversed by transferring the bulbs to the retarding temperatures of 25° and 30° again. If the retarded bulbs are transferred to higher temperatures than the storage temperatures, for instance to 40°, then a steady increase of the respiration rate is also observed, but there is no question of a steady state. The increase at 40° is very decided and potentially reversible under certain conditions. This effect is called the 40° effect. Shootless bulbs also show this phenomenon, but the effect is lost when experiments with bulb scales that are cut loose are made. If the dry membraneous tunics, which surround the bulbs, are peeled a pronounced rise in the respiration rate of the bulb occurs (the tunic effect). The hypothesis is put forward that this activity is similar to the 40° effect. It was tentatively deduced from the experiments that this activity is located in cell-layers near the convex side of the outer (first) bulb scales. The low respiratory activity of retarded bulbs at 25° is neither a result of a low oxygen tension normally existing inside the bulb, nor the result of a water loss of the tissues. Nor is it likely that the sugar as a substrate limits the respiration process. No changes of the sugar content in the bulbs were observed after some days’ temperature treatments. The controlling mechanism of the respiration rate is as yet unknown. The data give some indications that the incipient growth of the primordia inside the bulbs is stimulated by an oxygen concentration that is lower than in air. One would expect an oxygen shortage to occur in bulky tissues like in bulbs, but we did not observe a lack of oxygen under normal circumstances in any experiment. It is true that the respiration quotient fluctuates to some extent; the R.Q., however, practically remains unchanged, even under conditions of a high respiration activity. The 15° effect, the 40° effect, the tunic effect, as well as the wound effect appeared to be obviously dependent on oxygen. Transition effects at sudden temperature changes from 25° to 40° and vice versa are described. These effects were termed the peak effect and the trough effect.