118 • Age and Flowering 



example, in grafting experiments by Paton and Barber (1955) and 

 Haupt (1958) on early and late (lowering in peas (see Chapter Five). 

 There are also plants in which the flower primordia, following a 

 certain number of leaves, are already present in the seed (see 

 Naylor, 1958). 



One should attempt to distinguish between minimum leaf 

 number, as in the case above, representing a condition in which 

 a certain amount of development takes place before and during 

 the treatments leading to flowering, and ripeness-to-flower under- 

 stood as a condition before which a given treatment is completely 

 ineffective in promoting flowering. In practice, such distinctions 

 may be difficult to make. If the treatment in question is vernaliza- 

 tion, however, it is clear that the difference between winter annuals 

 and biennials (Chapter Five) simply reflects the fact that the latter 

 are not responsive until they have attained a considerable size. In 

 this sense, some winter annuals are ripe-to-flower as germinating 

 seeds. The reason for the size requirement in biennials is not 

 known, and has been ascribed to many factors, including the 

 amount of food reserves. De Zeeuw and Leopold (1955) found that 

 the age at which seedlings of Brussels sprouts, Brassica oleracea 

 gemmifera, a biennial, could be vernalized was decreased if the 

 synthetic auxin NAA was given together with the cold treatment; 

 the effect was not great, so that evidence that the size requirement 

 in biennials is related to auxin content is scanty. 



A series of experiments by Sarkar (1958) on a winter-annual 

 strain of the crucifer Arabidopsis thaliana illustrates not only the 

 complexity of possible relationships between development and 

 receptivity to cold treatment, but also the fact that the cold treat- 

 ment itself may have a multiple action, as evidenced by the ability 

 of gibberellin to replace it at some stages but not at others. The 

 strain of Arabidopsis in question is easily vernalizable in the seed, 

 during germination, or in the mature rosette stage. Young rosettes 

 are less easily vernalized. Gibberellic acid, however, is most effective 

 on the young rosettes, less so on the older, and totally ineffective 

 on seeds. 



Many studies bearing on ripeness-to-flower deal with respon- 

 siveness to photoperiod. In certain plants, of course, previous 

 vernalization is a major factor aflecting such responsiveness and 

 thus also ripeness-to-llower in this sense. Since this relationship was 



