Went 



■149 — 



Thermoperiodicity 



The rhythmical development in tulip and hyacinth is controlled by varia- 

 tions in temperature, so that their strictly yearly cycle is synchronized with 

 the progress of winter and summer in the temperate zones where they thrive. 

 In Hippeastrum, an inhabitant of tropical regions with even temperatures 

 throughout the year, an autonomous rhythm of the meristem causes a regu- 

 lar sequence of initiation of 3-4 leaves after which a flower stand is formed, 

 after which, again, 3-4 leaves are produced, etc. This sequence can not be 

 changed by temperature treatment. 



0" 15» 



IJ" 



I7» Z0° ZS ii-S" If 01° 



J5° 



Fig. 3.- — Actual development (ordinate: condition of growing point, stage I being 

 vegetative, VIII being complete flower initiation) of the growing point in tulip and 

 hyacinth, as a function of storage temperature (abscissa, in degree centigrade). For 

 the tulip the broken curve is recorded after 4 weeks, for the hyacinth the solid curve 

 represents 8 weeks development at the different temperatures iirom Blaauw, Luyten 

 and Hartsema 1930, p. 51). 



One of the most interesting results of Blaauw's work is, that from the 

 curves of figure 3 it can be deduced that development can be arrested by both 

 low and high temperatures. The cessation of development near freezing is 

 not amazing, since it occurs in most plants. But all growth can be stopped 

 by keeping bulbs at 35 °C., which temperature is not injurious at all to the 

 bulbs. As soon as the temperature is lowered to a proper one for the stage 

 of development, growth is resumed as if no interruption has occurred. In a 

 series of experiments bulbs were inhibited for 6 months by either high or low 

 temperatures and then shipped to the Southern hemisphere. In this way 

 their rhythm was shifted half a year, and the bulbs kept time with the shifted 

 sequence of seasons, enabling successful shipping to the opposite hemisphere, 

 which had not been possible before (Blaauw, Luvten and Hartsema 

 1930). 



From a practical standpoint this spectacular success is not the most im- 

 portant. Of greater significance for the bulb industry is the fact that some 

 of the most important stages in the development of a bulb are passed during 

 its storage period. This had been realized by practical growers since Dames 

 in 1909, but it was Blaauw who furnished the theoretical background and 

 who rationalized the treatment. Adjustments to the treatment in storage 

 are possible, which improve the later performance of the bulb in the field. 

 If the bulbs have to be planted in a rather warm climate, part of the cold 

 treatment they need (and usually receive in the field in colder climates) can 



