PHOTOTHERMAL AND CHEMICAL CONTROL 353 



ilar to that reached by aged plants even on SD (i.e., it flowers only 

 after a long time and after having produced a great number of leaves). 

 It is interesting to note that its growth behavior is quite different, as 

 the plant responds quickly to f-rLD with a vegetative bolting. To this 

 type, which corresponds to a standard one, belongs also a strain of 

 Crepis leontodontoides, of our test material. This also behaves, how- 

 ever, as a quantitative LDP. It differs from Centaiirea as it does not 

 elongate under f-rLD. 



A second type was found which, on the contrary, flowers under 

 f-rLD and remains vegetative on rLD. An annual strain of Brassica 

 napus, cultivated by us, behaves in this way. Probably many crucifers 

 of Funke's Group IV belong to this class. 



A third type with a further different reaction is exemplified by 

 Mimulus luteus. This species (or at least the strain used by us) flowers 

 both under rLD and f-rLD, although more rapidly under the red. We 

 have observed a similar behavior in other plants, as Aethusa cynapium. 

 This latter flowers better under f-rLD. 



After similar results, it seemed clear that very consistent differences 

 exist among plants of the same standard category. We cannot consider 

 the manifold aspects of this fact here, but it is well to be aware that 

 we can no longer depend upon the uniformity of responsiveness to 

 chromoperiodical conditions of plants pertaining to a standard pho- 

 toperiod type. For instance, many LDP that react quite similarly to 

 an extension of white incandescent light may behave very differently 

 toward the red and far-red extension. Their reaction may consequently 

 be different also toward various sources of artificial white light if the 

 red and far-red components are differently represented in them (Lona, 

 1957b). 



What we have just referred to is perhaps a picture that, besides being 

 limited, is simplified with respect to the actual situation. 



Stolwijk and Zeevart (1955) have observed that the LDP Hyo- 

 scyamus niger does not flower, or flowers with difficulty, when culti- 

 vated with a red light extension, while it flowers quickly with a far red. 

 Worthy of remark, on the other hand, is Down's observation (1956) 

 that Hyoscyamus niger flowers well when long nictophases are inter- 

 rupted by a flash of red light while, if this is immediately followed by a 

 far-red irradiation, the red action is hindered. In connection with these 



