80 DIVERGENCE OF VARIATION [ch. ix 



Cannaceae. Three outer staminodes separate. 



Two outer united, third free. 

 Dicotyledons. 



Casuarinaceae. Twigs whorled, rarely 4-angled, and then 



hairy in fork. 

 Twigs not whorled, or 4-angled with whorls 

 of four leaves. 



Myricaceae. Female flower with two to four or more 



bracteoles, not accrescent to fruit. 

 Female flower wdth two lateral bracteoles, 

 accrescent to fruit, making two wings. 



Myzodendraceae. Male flower with two stamens. 



Male flower with three stamens. 



Grubbiaceae. Flowers in threes in axils of foliage leaves. 



Fruit hairy. 

 Flowers in threes in axils of opposite bracts. 

 Fruit not hairv. 



Ceratophyllaceae. Fruit without spines or wings. 



Fruit with spines or wrings. 



Moringaceae. Seed without wings. 



Seed with wings. 



Nepenthaceae. Seeds egg-shaped with no appendages. 



Seeds with long hairlike coat. 



Myrothamnaceae. Two bracteoles. Stamens free. 



No bracteoles. Stamens in column. 



Platanaceae. Leaves usually 5-nerved. 



Leaves usually 3-nerved. 



Both in the monotype and the ditype families it will be seen at 

 once that the characters that distinguish the species in the one 

 and the genera in the other, are of the " family " type rather than 

 of the specific or generic type found in large families. And most 

 often they allow of no intermediates. Nothing but divergent 

 mutation will explain such things. 



It is fairly clear that the larger genera tend to head sub- 

 families, or groups of whatever rank may be considered appro- 

 priate in the family concerned. It will therefore be of interest to 

 study one or two families in greater detail, and the first that 

 comes up in a random choice is the Ranunculaceae. We shall 

 expect, upon the theory of diff'erentiation or divergent mutation 

 which we have been discussing, that the chief division in the key 

 will usually lead to the two chief groups into which the family is 

 divided, and that each of these will be headed by one of the two 



