THE PRINCIPLE OF NUMBER. 17 



subsequent differentiation in the foliage to an alternate 

 condition, the quaternary condition of the flowers remaining 

 unaltered. 



Similarly with the last condition, I found 212 genera of 

 30 orders with a quinary arrangement of the flowers corre- 

 lated to an opposite condition of the leaves, this being an 

 apparent anomaly of the same kind, but which is, however, to 

 be interpreted in the same way. Thus the Lahiatce are con- 

 stantly 5-merous in the flowers, but with as constantly 

 opposite leaves. Now, if we contrast this order with Scro- 

 plndarinecB, we find a similar constancy in certain genera 

 only, as in MMnanthus, etc. ; while other genera have alter- 

 nate leaves as Linaria, Digitalis, etc. 



There is an alternative of interpretations of this fact, for 

 both can be illustrated in nature. Either all the pentamerous 

 flowers have been deduced from alternate leaves (as may 

 have been the case with Mhinantlius and Lahiatce), the leaves 

 having subsequently reverted to the original or ancestral 

 state of opposition ; or else, the 5-merous character of the 

 flowers has arisen by a sudden change (possibly due to the 

 stimulus of insect agency) from opposition in the leaves or 

 bracts to an alternate arrangement in the parts of the flower. 

 As an illustration of this latter process may be mentioned 

 the development of the five sepals of Beutzia as compared 

 with the four of the allied genus Philadelphus. In this 

 latter genus the anterior and posterior sepals appear 

 together, subsequently the two lateral arise simultaneously. 

 In Deutzia, however, the two anterior sepals correspond to 

 Nos. 1 and .3 ; two sepals are lateral, viz., Nos. 4 and 5 ; 

 and the posterior sepal is No. 2. Thus the opposite and 

 decussate pairs of sepals of Fhiladelphus would be repre- 

 sented by the figures 1 and 2, 3 and 4. If these were to 

 break up into a quincuncial spiral and shift their positions, 



C 



