446 A NATURALIST IN THE PACIFIC chap. 



wither, and in a few days the flower becomes detached and drops 

 off. With Rhizophora mucronata and Rhizophora mangle, the 

 style preserves its healthy condition, and shortly evidences of 

 fertilisation appear in the altered shape of the ovary. It is apparent, 

 therefore, that in the case of the Selala fertilisation has not occurred, 

 although the mechanical processes connected with it have been 

 carried out. The cause of this is not far to seek. 



Although the ovaries of the Selala contain four ovules, which in 

 size and appearance do not differ from those of Rhizophora mangle 

 and R. mucronata, its pollen when compared with that of the other 

 two forms presents a remarkable difference. The pollen of these 

 three mangroves was examined in five localities far apart from each 

 other, and in all the same results were obtained. The pollen-grains 

 of the Selala are much smaller than those of the other two, and 

 differ much from them in form. They are irregularly oval in out- 

 line, and have a shrunken look beside the spherical symmetrical 

 grains of the two species with which they are compared. They are 

 from one-fourth to one-third the size of those of Rhizophora 

 mucronata, and from one-third to one-half the size of those of 

 Rhizophora mangle. 



There is much to support the view that the Selala is a cross 

 between the other two species, its intermediate characters and its 

 seedless condition being especially indicative of such a derivation ; 

 but there are several difficulties in accepting this explanation. 



(i) The circumstance of the anthers bursting in the flower-bud 

 would considerably lessen the chances of cross-fertilisation ; but 

 this objection is not insurmountable, since numerous insects, such 

 as flies, ants, and small coleoptera, visit the newly opened flowers, 

 and they might sometimes produce a result. When I made this 

 suggestion to Prof Schimper he replied that insect-pollination 

 was quite possible after the expansion of the flowers. 



(2) If, as seems highly probable, the pollen of Selala is impotent 

 and the ovules fertilisable, then its seedless condition implies not 

 only an incapacity for self-fertilisation, but also for cross-fertilisa- 

 tion ; and if Selala with its impotent pollen does not admit of cross- 

 fertilisation, this would still less be expected of Rhizophora 

 mucronata and R. mangle where the pollen is potent and where 

 fertilisation takes place in the bud. I endeavoured to fertilise the 

 Selala flowers with the pollen of the two other species ; but there 

 were no results, the flowers falling off in a few days. It may here 

 be remarked that on one Selala tree I found a solitary flower 

 with an enlarged ovary, as if through fertilisation. 



