THE MANY RESPONSE TYPES 13 



These he combined with an absolute requirement for long-days, a 

 partial requirement ("great" or "small"), no requirement for long- 

 days, and a requirement for short-days. Considering the "great" and 

 "small" requirements, there were 20 possible response types, and he 

 cited examples of plants which are known to belong to each of the 

 20 groups. 



Chouard's table of response types omitted a number of categories 

 such as the quantitative short-day plants and the intermediate and 

 combination-requiring plants. There is also no mention in the table 

 of temperature and day-length interactions. Considering only what 

 has been discussed above, it can be seen that any chart purporting 

 to classify the many response types should have at least three dimen- 

 sions : one for the day-length response, one for temperature response, 

 and one for the interactions. This is the approach followed in the 

 rest of this chapter. 



It is very easy, however, to think of other dimensions which could 

 be added to such a chart. We will ignore these because as a rule, 

 they are known to be of importance only in isolated cases. Light 

 intensity, for example, is known to influence flowering in an absolute 

 way in certain species; that is, flowering occurs only when the light 

 intensity reaches a certain level (high in some cases, low in others). 

 Light quality is known to be of paramount importance in virtually 

 all of the plants which respond to day-length, and there are certainly 

 many kinds of response to light quality. For example, with Sahia, 

 the proper mixture of wavelengths during the day caused a red light 

 interruption of the dark period to promote instead of inhibiting. 

 Some of these responses to light quality are discussed in Chapter 7, 

 but they will not be used in the classification which is attempted here. 



In the early days of plant physiology, mineral nutrition was thought 

 to play a very important role in flowering of all plants. It was 

 theorized that the nutrient status of the soil would control the 

 flowering of certain species. Since the work of Garner and AUard, 

 however, it has been shown that the soil seldom exercises any real 

 control on the flowering process. It is true that quantitative responses 

 are easy to demonstrate : high nutrient levels in the soil promote the 

 flowering of cocklebur (see Table 5-1 in Chapter 5), but the cocklebur 

 will not flower at all unless the day-length conditions are right. Low 

 nutrient conditions seem to promote the flowering of other species, 

 and such knowledge has considerable horticultural application. This 



