18 THE STRUCTURE OF FLOWERS. 



they would, with the addition of one more sepal, assume 

 those represented by Deutzia. 



2— > 



Exactly the same procedure occurs in the change from 

 opposite to alternate arrangements of leaves in the Jeru- 

 salem Artichoke, as I have explained in treating of the 

 varieties of leaf- arrangement in that plant. 



Calycanthus is another instance illustrating an abrupt 

 change from an opposite condition of the leaves to the ^V 

 type in the bracts enveloping the flowers, and which then 

 pass insensibly into sepals and petals. 



Symmetrical Increase and Decrease in Floral Whorls. — 

 As another instance of variability adding further complica- 

 tions, it may be observed that in both kinds of arrangements, 

 namely, of those plants possessing alternate and those pos- 

 sessing oi3posite leaves, there are many genera whose floral 

 symmetry ranges from one to some higher number in the 

 different species of the same genus. Thus 4-5-merous flowers 

 are especially common. I found it so in more than 100 genera 

 of 23 orders examined among alternate- leaved plants ; and 

 58 genera of 19 orders among those with opposite leaves. 



Again, some genera have species the whorls of whose 

 flowers range from 3 to 5 or 6, or from 4 to 6 in the number 

 of parts ; others from 5 to 7 or 5 to 8, etc. In these 

 cases it is often quite impossible to explain what has been 

 the immediate causes producing such variations. The only 

 interpretation that can be given is that the primary sym- 

 metry having been originally determined by phyllotaxis, it 



