Went — 147 — Thermoperiodicity 



for 3 weeks, after which it stays around 9°C. During those weeks the 

 flower parts develog into a complete flower, and the stem elongates slightly. 

 When the direct effect of temperature is measured by actual stem elonga- 

 tion, the optimal temperature is higher, but such higher temperatures re- 

 tard subsequent elongation, compared with the 9° temperature. There- 

 fore Blaauw speaks here of an indirect or inhibited optimum. It com- 

 pares with the low vernalization temperatures, which inhibit germination, 

 but accelerate later growth. By the time the leaves become visible from be- 

 tween the scales, the optimal temperature shifts to 13°C., which is optimal 

 for actual stem elongation, and the optimum shifts still further to 17°C., 

 when the leaves have emerged 3 cm. from the bulb. When they are 6 

 cm. long the optimal temperature shifts again, to 23 °C. This means that 

 each developmental stage has its own optimal temperature; organ initia- 

 tion needs the highest temperature, stem elongation and unfolding of the 

 flower a lower one, and preparation for elongation, a stage which has no 

 morphological signature, occurs best at the lowest temperature. It is likely 

 that the steps between 9°, 13° and 17° are not abrupt, but gradual, so that the 

 stippled curve of figure 1 probably approximates the actual conditions closer 

 than the step-curve. The drop from 20° to 8° on the other hand is abrupt, 

 and any intermediate temperature interposed between these two delays de- 

 velopment. 



For the hyacinth a slightly different curve was obtained, as seen in fig- 

 ure 2 (from LuYTEN, Versluys and Blaauw 1932). The whole curve 

 lies about 4° higher than that of the tulip. But most important is the dif- 

 ferent behavior during the early weeks. As in the tulip, the highest optimal 

 temperature occurs immediately after lifting, and in this case is 34 °C. By 

 the time the first flowers on the raceme have been initiated the optimal tem- 

 perature shifts to 25.5°, and when the highest flower are visible as primordia 

 on the raceme meristem, the optimal temperature drops to 17°. Three 

 weeks later the lowest optimal temperature of 13°C., is reached. When 

 the high temperature of 34° is maintained throughout the period of flower 

 initiation, subsequent flower development is abortive, and racemes with flow- 

 ers of poor quality are produced. Therefore the intermediate temperature 

 of 25.5° is a compromise between the optimal temperatures of at least two 

 different processes, which proceed simultaneously inside the bulb: flower 

 initiation with a very high temperature optimum, and preparation for further 

 flower development, with a much lower optimum. 



Before discussing these results any further, another paper from the same 

 laboratory, by Versluys (1927) must be mentioned. She studied the op- 

 timal temperatures for root initiation and root elongation throughout the 

 whole development of the hyacinth bulb. It was found that the optimal 

 temperature for root growth remained approximately constant at 27° C. 

 During the part of the life cycle of the bulb which was investigated, few new 

 roots were initiated, but the greatest number formed occurred again at about 

 27°C. 



Viewing these results together, it can be said that each physiological 

 process in the hyacinth bulb has its own optimal temperature, which dif- 

 fers from that of other processes. Therefore the over-all optimal temperature of 

 the whole bulb is a compromise between the optima of the individual proc- 



