THE PRINCIPLE OF NUMBER. 10 



rhjinges, wliether in the individual or in its descendants, 

 through the law of " symmetrical increase or decrease." By 

 this 1 mean that the number of sepals, petals, and stamens 

 often vary together from the typical number by the addition 

 or subtraction of a member. Thus, in a single corymb of 

 an Elder, 4-, 5-, 6-merous flowers may be often found; simi- 

 larly, while early blossoming Fuchsias may bear 3-merous 

 flowers, they are replaced later by the regularly 4-merous 

 ones. Although these changes frequently occur in the same 

 plant, they usually are not permanent. Yet they occasionally 

 appear to have become so, as in the terminal flow^ers of Adoxa 

 and Monotropa. On the other hand, the constant occurrence 

 and, therefore, specific character of 4-merous flowers in 

 Fotentilla Tormentilla, and 3-merous in Tillcea muscosa, I 

 should be inclined to attribute to the fixation of a symme- 

 trical reduction which has taken place from the permanent 

 5-merous type so characteristic of Potenfilla, and many 

 genera, of the CrassidacecB. Not infrequently the difference 

 of number is pronounced by systematists as generic ; thus, 

 while Riihia has 5-merous flowers, Galium has 4-merous. A 

 similar difference lies between Ruta and Ha'plopliyllum* 



If a cause be looked for, it would seem to be merely a 

 question of nutrition. If the symmetry varies in the same 

 plant, it is obvious that a corolla of four petals could not 

 have been provided with the same amount of nutritive 

 material as a 5-merous one. But if it be a specific character, 

 as in Tormentil (which, it may be observed, affects the more 

 or less barren soil of heaths), then the change has become 

 fixed and is now hereditary. 



* By running the eye throufjh the artificial keys at tlie commence- 

 ment of the Orders in the Genera Plantarum of Bentham and Hooker, it 

 Avill be seen how frequently these authors regard the p umber of parts in 

 the Calyx and Corolla as a prominent generic character. 



