Vemalizatioii and Photoperiodism 



■146 — 



A Symposium 



Another reason for the importance of Blaauw's work are his micro- 

 scopic observations of the meristem during and immediately after each tem- 

 perature treatment, so that he could distinguish between the direct and in- 

 direct temperature effects, and so that he could identify each optimal tem- 

 perature with a definite morphological stage. 



After more than 10 years of work along these lines Blaauw (1931) 

 concludes : "It is hoped that the time has passed, when the development of 

 buds and defoliation are described without any knowledge of what goes on 

 inside the bud, and the development of the leaf primordia, of which only 

 their macroscopical appearance is being observed." "Before talking about 

 periodicity and dormancy, in connection with the later behavior of sprout- 

 ing leaves or flowers, it is imperative that first the life history of these 

 organs inside the bud is studied. For this precedes sprouting." 



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Fig. 1. — Optimal temperatures (ordinate, in degree centigrade) of the development of 

 tulip bulbs (var. W. Copland) from the time of lifting from the ground to flowering (abscissa; 

 time in weeks). Step-curve: experimentally determined optimal temperatures. Stippled 

 curve: most likely actual optimal temperatures {.from Hartseua, Luyten and Blaauw 

 1930, p. 33). 



In the experiments dry bulbs were placed in baskets and stored in tem- 

 perature controlled rooms and incubators. All treatments were started im- 

 mediately after the bulbs were lifted from the ground around July 1, when 

 the current year's leaves had withered, and when the bulb seemed to have 

 entered dormancy. 



As a first example the development of the tulip bulb will be discussed. 

 Figure 1 is taken from the publication of Hartsema, Luyten and Blaauw 

 (1930). Immediately after lifting, at the beginning of the experiment, next 

 season's growing point has differentiated 3-4 leaf primordia, and is almost 

 ready to initiate the flower. At that time the optimal temperature is high- 

 est, 20°C. When all flower part primordia have been initiated (about 3 

 weeks) the optimal temperature drops abruptly to 8°C., where it remains 



