8 The Irish Nahiralist. January, 



Kerry, and a total combined flora of 855. Mr. Colgan's 

 "index" is therefore ^f--, or -296. But if, instead of "296 



T 7^ I »iQ 



we express the relation as g^.^ j or for sake of clearness 



— ^'^ g '_ — ^, A standing for Antrim and B for South Kerry, 



all the facts of the case stand out clearly ; whereas '296 might 

 mean that there were 253 plants peculiar to Antrim, and no7ie 

 peculiar to Kerry. I fancy, however, that in practice two 

 formulae will be used by the botanist who takes up this branch 

 of plant-distribution, namely, Mr. Colgan's "index" for ex- 

 pressing the floral diversity of two areas apart from the con- 

 stituent factors ; while the simple formula a^b-\-c, where a 

 expresses the number of species common to both, and b and c 

 the number peculiar to each, will give all the further informa- 

 tion required. Thus, in the case above quoted, the formula 

 Antrim -f South Kerry = 602-1-175-1-78 gives a full numerical 

 comparison of the floras. Mr. Colgan's ** index," expressed in 



these terms, is, of course, , , ^ . 



IS THE HOLLY DICECIOUS? 



BY THOMAS COOKE-TRKNCH. 



In The Irish Nahwalist for May, 190I;. I raised the above 

 question, and it may be of interest to the readers of the 

 Journal to learn the result of my experiments on the subject. 



It will be remembered that Darwin, while stating that of 

 many plants examined, he had never found one that was really 

 hermaphrodite, admits that of the several authorities that he 

 had consulted one only states that the Holly is dioecious. 



The experience of a correspondent of my own, living in the 

 New Forest, agrees with Darwin's. My friend marked a number 

 of berry-bearing Hollies in the Forest, and subsequently 

 brought home and carefully examined the flowers of these, 

 but in no case was pollen found in the anthers. 



An anonymous friend has sent me the following extract 

 from Willis' "Flowering Plants and Ferns," Vol. II. :—" //rjr 

 Agiiifoliiwi, Flowers dioecious, but in the female flower the 



