Temperature and Flowering in Bulb Plants • 65 



collaborators in the Netherlands, particularly in the period 1920- 

 1935, on the initiation and development of flowers in bulb plants. 

 This work is largely recorded in Dutch but has been reviewed by 

 Went (1948), from whom this account is taken. The basic pro- 

 cedure was to store bulbs at different temperatures for different 

 lengths of time and determine, by anatomical studies, the optimum 

 temperature for the various developmental events taking place 

 within them. 



After the current year's foliage has died, the next year's apical 

 meristem within the tulip (Tulipa) bulb already has several leaf 

 primordia. Flower initiation, including differentiation of all the 

 flower parts, then takes about three weeks at 20° C, the optimal 

 temperature for this process. If further flower development is to 

 take place (still entirely within the bulb), the temperature must 

 now drop and remain at about 9° C for 13 to 14 weeks. After this 

 low-temperature period the optimal temperatures for leaf and stalk 

 elongation are successively higher, reaching 20° and above for com- 

 plete anthesis. This increase in optimal temperature for the final 

 stages of flowering is more or less gradual, but it appears to be 

 characteristic of tulip and certain other plants that flower initiation, 

 favored by relatively high temperatures, must be followed quite 

 abruptly by low temperatures for the best subsequent development. 

 In the hyacinth (Hyacinthus) bulb, on the other hand, the changes 

 in temperature optima are not as abrupt as in the tulip, though they 

 are similar, and all the values lie somewhat higher. 



Such studies have since been conducted, in the Netherlands 

 and elsewhere, on many plants having bulbs, rhizomes, or other 

 fleshy organs that can be stored for a considerable part of the year. 

 The detailed results of course differ from plant to plant, but are 

 usually of great practical value since they make it possible to 

 control development or arrest it at desired stages to suit almost any 

 shipping and planting schedule. Tulips and hyacinths, for example, 

 can be held completely dormant without injury for weeks by 

 storage at 35° C. As soon as further development is required, the 

 temperature can again be lowered to the optimal level for the stage 

 previously attained. Recent references to this sort of work can 

 be found in journals and textbooks on horticulture. 



It needs to be stressed that this sort of temperature response is 

 not characteristic of all bulb plants, but merely of those adapted to 

 temperate climates with a well-defined winter. The tropical bulb 



