76 



THE FLOWERING PROCESS 



Strain of plants is adapted to the hot summer days and nights which 

 prevail around their native Chicago. 



The best temperature conditions will obviously depend strongly 

 on the species being used (Chapter 2). Probably an alteration in 

 temperature between day and night is best for most plants, but we 

 applied this principle to the cocklebur for a decade, and it was 

 probably responsible for most of the difficulties encountered with 

 our winter experiments ! Of course, optimum temperature conditions 

 should be determined for any new species to be investigated. 



5. Other Greenhouse Procedures 



In our methodology we keep plants trimmed, so that only the 

 relatively young leaves are left on the plant at any time. When plants 

 have produced four or five leaves longer than about 1 cm, all but the 

 first one or two are removed by trimming. This makes subsequent 

 steps in our procedure easier, and it also seems to cause the plants to 

 reach a mature stage at an earlier date. 



Cocklebur plants flower in response to the long night much better 

 if they are provided with an optimal supply of nutrients in the soil, as 

 shown in Table 5-1 . The uniformity of response is greatly improved. 



Table 5- 



Effects of Fertilizer upon Flowering and Uniformity 

 OF Floral Stages in Cocklebur 



1 The Floral Stage system is described at the end of this chapter. 



2 Plants given one 16-hr inductive dark period, Oct. 31, 1960. Previously trimmed 

 to young No. 3 leaf. Buds examined after 9 days. Plants dipped in solution 

 containing 3 drops wetting agent (Vatsol)/250 ml just before the dark period. 



3 Fertilized with \ teaspoon of Ortho-Gro 16-16-8. 

 * Standard deviations. 



