Vernalization and Photoperiodism 



— 148 — 



A Symposium 



esses, as was clearly expressed in the flower-stand formatior. Since not all 

 flowers are initiated at the same time, some are already so far advanced that 

 they need low temperatures for further development, while other flowers still 

 are in the process of initiation, and consequently require high temperatures. 

 While the later stages of elongation proceed best at 13°C., the temperature 

 should be 27 °C. for best root growth. Since root development does not 

 seem to be controlling bulb growth, and sufficient roots are formed at the 

 optimal temperature for elongation, no compromise between root and stem 

 elongation temperatures is necessary. 



34' 



25.5" ; 

 17° 



Initialion of 

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Organs 



Elongahon 



io 



iS 



S Weeks '* 



Fig. 2. — Optimal temperatures (ordinate, in degree centigrade) during the develop- 

 ment of hyacinth bulbs. Abscissa: time in weeks since lifting of bulbs from the ground 

 (.from LuYTEN, Versluys and Blaauw 1932, p. 51). 



Whereas the curves shown in figures 1 and 2 show the shift in optimal 

 temperatures, many diagrams showing the progress of individual processes 

 at diff^erent temperatures are found in Blaauw's papers, and figure 3 (from 

 Blaauw, Luyten and Hartsema 1930) shows a graphical presentation of 

 the flower development as a function of temperature. It shows the range of 

 temperatures in which flower development can proceed with an optimum 

 about 12° wide. The curves of figure 4 (from Blaauw 1924) show the 

 shift in optimum when observations are made in different intervals. From 

 such curves it also would be possible to construct the curve of figure 2, but 

 it would be less accurate. 



For other bulbs the same type of an analysis was carried out. Daffodils 

 (variety King Alfred) show a behavior similar to that of tulip and hyacinth, 

 except that flower initiation already has taken place in the field, so that the 

 optimal temperature starts as low as 13°C., lowering to 11° after eight 

 weeks and shifting to 10° when leaves become visible. After 2 weeks the 

 optimum increases to 17° and when leaves are 6 cm. to 20°C (see Blaauw, 

 Hartsema and Huisman 1932). In stark contrast with the bulbs from 

 temperate climates the tropical Hippeastnim (Blaauw 1931) has no ob- 

 vious resting period but 2 to 3 times per year a whole cycle of leaf and 

 flower formation is completed, at completely even temperatures. 



