THE MANY RESPONSE TYPES 21 



not given low temperature may be day-neutral, but 

 follomng treatment with low temperature, long- 

 days (or short-days) are required for flowering. 



Age at Time of Response and Flowering Time 



In addition to the above dimensions it is convenient to know the 

 condition of the plant at the time when it responds to the environ- 

 mental stimulus and also to know the time when the plant normally 

 flowers in nature, that is, whether it is an annual, a biennial, or a 

 perennial. These factors will not be taken into consideration in the 

 classification, although they are of considerable importance especially 

 in the case of vernalization. The following lists indicate these factors 

 along with the symbols which are used : 



Plant Type : (Condition at sensitive time.) 

 i 1. Seed or young seedling. 



i 2. Rosette plants (leaves all arising from the root, such as a 

 dandelion), including grasses. 



3. Caulescent plants (leaves from a stem). 

 I A. Herbaceous. 

 t B. Woody (bushes or trees). 



Flowering Time : 



1. Annuals. 

 y A. Winter. 



I B. Spring. 



ij C. Summer. 



l^ 2. Biennials. 

 I 3. Perennials. 



The Categories of Response Type 



Having listed the responses to photoperiod, temperature, and the 

 interactions, it should now be possible to work out the theoretical 

 combinations of these factors and discover how many response types 

 might be expected if all combinations occur in nature. This can be 

 done by use of a three-dimensional approach such as that shown in 



