14 THE FLOWERING PROCESS 



information could be taken into account in a classification system. 

 In general, however, experimentation has not proceeded far enough 

 to allow us the use of such criteria in our classification. 



One species is known which seems to respond in an absolute way 

 to soil conditions. This is the sugar cane, which in Hawaii will flower 

 profusely when it is grown upon one type of soil, and remain 

 absolutely vegetative in a near-by field on a different soil type (45). 

 The same response is obtained when plants are grown on the two 

 soils in pots in the greenhouse. 



Still another dimension might be the age of the plant. Datura (the 

 jimson weed), for example, is promoted in its flowering by short-days 

 when it is young, but in its old age it flowers equally well on any 

 day-length. Since the age of a plant is much more important in some 

 species than in others, this could also be used as a criterion in 

 classification. 



The Approach to the Classification System 



The following classification system is restricted to conditions 

 which might be encountered in nature : combinations of day-lengths 

 obtainable on a 24-lir cycle and temperatures which might be 

 encountered during the normal day and night in nature. The responses 

 to colored lights, interruptions of the dark-period, short interruptions 

 with extreme temperatures, etc., are not used. These will be discussed 

 later when we investigate the mechanism of the flowering process. A 

 classification system for each of the three dimensions listed above is 

 outlined, and some combinations of these responses known to have 

 examples among the flowering plants are listed, along with examples, 

 in an appendix. 



Two things were found to be of considerable help in working up 

 the outline. One was a system of symbols which allows one at a 

 glance to know the response type of any given species. The symbols 

 are listed along with the response types in the classification below and 

 in the appendix. Another handy mechanical help was a punched 

 card filing system as illustrated in Fig. 2-1. 



Before going to the classification system, one more point should be 

 emphasized again. A short-day response is not simply a response to 

 "short" days. The response (on 24-hr daily cycles) is to day-lengths 

 which are shorter than a maximum or night lengths which are longer 



