96 THE STOKY OF THE PLANTS. 



soil, or under cultivation. This is what we call 



" doubling " a flower. In the double rose, for 



example, the extra petals are produced from the 



stamens of the interior, and if you examine 



1 them closely you will see that they often show 



every possible gradation and intermediate stage, 



I from the perfect stamen to the perfect petal. 



The same thing readily happens with buttercups, 



poppies, and many other flowers. We may take 



it for granted, then, that petals are, in essence, 



a single outer row of stamens, flattened and 



coloured, and set apart by the plant to advertise 



its honey to insects, and so induce them to visit 



and fertilise it. 



In the largest and most familiar group of 

 flowering plants, to which almost all the best- 

 known kinds belong, the original number of 

 petals seems to have been five ; and we will take 

 this number as regular for the present, explain- 

 ing separately those cases where it is exceeded 

 or diminished. The common ancestor of all 

 these plants, we may conclude, had all its parts 

 f in rows of five. Thus it had five, ten, or fifteen 

 ' carpels in its pistil that is to say, one, two, or 

 'three rows of five carpels each; it had five, ten, 

 or fifteen stamens, it had five or ten petals, and 

 it had a calyx, outside all, of five sepals. We 

 will now proceed to examine in detail some of 

 the many curious marriage customs which have 

 arisen among the group of plants that started 

 with this ground -plan. 



One great family of plants which early divided 

 itself from this great central stock is the family 



