94 THE FLOWERING PROCESS 



species grow for 5 to 50 years or longer, flower once, and then die. 

 Presumably the environmental stimulus to flower, if one is required, 

 is received each year, but the machinery to respond to it is not 

 developed until the plant has reached ripeness to flower. Often it 

 must grow beyond a so-called juvenile stage to one of relative 

 maturity. 



The juvenile stage is itself an interesting topic for discussion (76). 

 Juvenile features are often exhibited in many respects besides the 

 inability to flower in response to environment, and often this feature 

 is not well correlated with the others. Leaf shape of the first leaves 

 may diff'er markedly from the shape of later leaves. This is fairly 

 well illustrated in Fig. 1-1-B for the cocklebur plant (here the juvenile 

 leaves have already reached ripeness to flower). Arrangement of 

 leaves on the stem (phyllotaxis) may also change during maturation. 

 The first two cocklebur leaves are usually opposite, while successive 

 leaves become more and more alternate. There are often changes in 

 the shape of thorns, changes in ability to form sun or shade leaves 

 or to form roots on cuttings, and changes in other features. Although 

 the change from the juvenile to the mature condition is usually 

 gradual, the two forms may be so different from each other that they 

 can easily be mistaken for diff'erent species. The two forms of ivy, 

 one a vine with palmate leaves and the other a shrub with entire 

 leaves, are actually treated as separate horticultural varieties. 



A striking feature of the change from the juvenile to the mature 

 stage is the resulting stability. Cuttings from adult parts of the plant 

 (the base of a tree will often retain its juvenile characters) maintain 

 their adult characters when they are rooted or grafted to young root 

 stocks. A famous example is a cutting made from a plagiotropic 

 (horizontally growing) adult branch of Araucaria excelsa which has 

 retained its horizontal growth habit for more than 50 years at the 

 Munich Botanic Garden. It appears that some basic change has 

 taken place in the tissue, and that this change is preserved through 

 cell divisions as the plant continues to grow. 



There is some evidence that hormones are involved in the manifes- 

 tations of juvenility. Perhaps most striking is the reversion of adult 

 ivy to the juvenile stage after treatment with gibberelUns (73). 



Other changes take place in growing plants which are not to be 

 confused with maturation, but which seem to be part of the ageing 

 process. It is well known that most organisms exhibit a sigmoid 



