THE LOW TEMPERATURE PROMOTION OF FLOWERING 69 



retarded or completely inhibited flowering. In most bulbs, the 

 optima for flower initiation were relatively high, but sometimes 

 fairly low temperatures were most effective (see below). Low tem- 

 peratures might also be concerned with breaking of a "rest period", 

 or they might be required for optimum stem elongation after flowers 

 begin to emerge (another gibberellin effect ?). The actual temperature 

 responses were closely correlated with the behaviour of the plant in 

 the field, and Hartsema divides these response types into seven 

 categories. These have been condensed and simplified somewhat into 

 the following four groups : 



A. Flower primordia form a year in advance, before bulbs can be 

 harvested. Obviously effects of bulb storage temperatures on flower 

 initiation cannot be studied in these species, since flowers are formed 

 long before bulbs can be stored. Photoperiod and temperature 

 conditions could be important during initiation, however, but these 

 factors have seldom been studied. In two tropical species, leaf and 

 flower formation for the next year go on simultaneously during the 

 entire assimilation period {Hippeastrum, Zephyranthes). In other 

 species flower formation for the next year also occurs before this 

 year's flowers are gone, but in a somewhat more regular fashion 

 {Amaryllis belladonna, Nerine sariensis). In others formation occurs 

 after blooming but before the bulbs can be harvested {Narcissus or 

 daff'odil, Galanthus, Leucojum, Convallaria). 



B. Flower primordia form during the storage period after harvest 

 in the summer but before replanting in the fall. Perhaps the most 

 thorough work has been done with plants in this category. A typical 

 curve showing optimal storage conditions for tulip is shown in 

 Fig. 4-5. Examples include Tulipa, Hyacinthus, and Crocus. Optimal 

 temperatures for flowering are relatively high (17-20°C for tulips, 

 25.5°C for hyacinths), although low temperatures are required for 

 early blooming and good stem elongation. 



C. Flowers are formed during the winter. If bulbs are planted in 

 the fall (most bulbous irises — see Fig. 4-5), then flower formation 

 occurs during the low temperatures of winter (9-1 3°C optimum), but 

 it was found that a high temperature (23-30°C) pretreatment was 

 essential if flower formation was to occur at all. This seems to be a 

 true inductive or delayed effect, very similar to vernalization except 

 that high temperatures are required instead of low. If bulbs are 

 stored during the winter, then flower formation occurs during storage, 



