96 THE FLOWERING PROCESS 



plantlets, which will not respond to photoperiod by flowering and 

 which have a long juvenile phase, and grafted them onto large 

 flowering plants. The small plantlets began to flower immediately. 

 Thus the meristems were capable of responding to the hormone but 

 the leaves on the seedlings apparently cannot respond to the 

 environment by producing hormone. 



Similar results have been obtained with a few other herbaceous 

 species, but as implied above, grafting of juvenile woody shoots onto 

 mature trees will not induce immediate flowering of the juvenile 

 shoots. 



2. Is Ripeness Attained by Counting Cycles or by Summating 

 Total Active Growth Time? 



K. A. Longman and P. F. Wareing in Wales (60) set up an experi- 

 ment in 1957 designed to solve this problem for birch (Betula 

 verrucosa) which requires 5 to 10 seasons to flower in nature. One 

 group of seedlings was grown continuously under 1 8-hr days in the 

 greenhouse ; another group was given long days until 25 to 30 cm of 

 growth had occurred, then short days (9 hr), causing the plants to 

 become dormant, 6 weeks of chilling to break dormancy and then 

 long days again, and so on. After 10 months the seedlings under 

 continuous long days began to flower when they were 2 to 3 m tall. 

 Within two years, 13 of the original 14 trees had flowered. None of 

 the other trees given 7 short, simulated seasons had flowered by then, 

 even though that many full seasons in nature would have probably 

 caused flowering. Thus ripeness to flower in this instance seems to 

 depend upon the total growth attained rather than the number of 

 cycles of growth and dormancy. 



3. What is the Physiological Nature of Ripeness to Flower (37) ? 



It is known that sugar is necessary for vernalization of winter rye 

 seedlings (see Chapter 4), and it has been suggested that ripeness to 

 flower is a matter of building up enough reserve carbohydrate. 

 Furthermore, the juvenile phase can be shortened in Lunaria biennis 

 (a biennial) by strong additional light which may increase photo- 

 synthesis and reserve material. In other biennials vernalization does 

 not take place after defoliation unless the plants have reserve organs 

 such as tubers. Isolated apical buds of carrot and beet can be 

 vernalized, but only in the presence of sugar solutions. Only one 



