18 THE STRUCTURE OF FLOWERS. 



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

 those represented by Beutzia, 



5 4 



4 3 14 5 



6 



Philadelphus, Deutzia, 



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. 



Calycantlius is another instance illustrating an abrupt 

 change from an opposite condition of the leaves to the gx 

 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 opposite 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 



