52 THE FLOWERING PROCESS 



roots had no effect unless the apical meristem of the shoot was 

 cooled. Such plants include onion, beet, henbane, and chrysanthe- 

 mum. With a cereal seedling, the embryo itself perceives the cold, 

 and not the endosperm. Thus it has been generalized that it is always 

 the meristem that responds to cold in vernalization. 



It was learned in 1955, however, that cotyledons of Streptocarpus 

 could be vernalized, and in 1961 Wellensiek found that detached 

 leaves oi Lunar ia biennis could be vernalized without buds (37). Such 

 leaves regenerate flowering plants, but controls not subjected to cold 

 produce only vegetative plants. In subsequent work he was able to 

 show that the leaves had regenerating tissue at their base, and thus 

 it was possible to suggest that vernalization requires dividing cells 

 if it is to be effective (see Chapter 10). Response of seeds below 0°C 

 suggests, on the other hand, that dividing cells are not essential. To 

 settle the problem, careful cytological work will have to be done. 



The meristematic cells of the growing point are changed by the 

 cold in some way which allows them to become reproductive at a 

 later date. The time between perception of the cold and actual 

 expression of the stimulus by appearance of flowers might involve a 

 fairly large portion of the life cycle. This is quite apparent for the 

 cereals, which respond to cold in the seed stage. Even in henbane, 

 if the cold treatment is followed by short-days the plant will remain 

 vegetative for an indefinitely long period of time (190 days in one 

 experiment for example) until it is finally converted to the reproduc- 

 tive condition by treatment with long-days. The meristem has 

 progressed towards flowering in response to the cold, but it produces 

 only leaves until long-days (presumably a stimulus sent from the leaf 

 in response to long-days) complete its transformation to the repro- 

 ductive state. True vernalization, then, provides one of the best 

 known examples of the induced state, a topic to be discussed again 

 in Chapter 10. 



3. Condition of the Plant 



As indicated in Chapter 2, the age of the plant at sensitivity depends 

 upon the species. In cereals, the damp seed itself will respond to the 

 cold, and immature seeds still on the mother plant have been 

 vernalized. Cereals usually are vernalized in the seed or seedling 

 condition, but it has been shown that mature cereal plants will also 



