Salvoza as separate species, mainly on the basis of their 
geographical separation into two groups living respective- 
ly on the east and west coasts of America, and because 
of a difference in length of style. Salvoza also recognized 
R. brevistyla Salvoza on the Pacific coast as a species 
distinct from R. Harrison Leechm. on the east coast 
of America, chiefly on the basis of a difference in flower 
and flower-bud size and difference in stylar length. Hou 
does not think that the difference between the two groups 
in each pair is sufficient to warrant specific designation. 
He recognizes only three American species: R. Mangle 
L., R. Harrison Leechm., and R. racemosa G. F. W. 
Meyer. Of these, only R. racemosa has not yet been 
found on the Pacific coast of the Americas, the other 
two occurring on both coasts. 
Pollen of the various New World species of Rhizophora 
had previously not been studied in detail. Van der Ham- 
men (1963; pers. comm., 1965) has expressed the opinion 
that pollen of the three species of Rhizophora occurring 
today in British Guiana can be distinguished morpholo- 
gically, but he does not separate them in his data. Muller 
(1959) observes that pollen of the Rhizophora-ty pe shows 
‘‘rather strong variation in size and in the appearance of 
pores.’’ He attributes this variation, however, to differ- 
ence in preservation rather than to morphological dis- 
tinction of value in separation of the species. 
Observations of modern Rhizophora pollen by the 
authors have led them to concur with van der Hammen 
that the species of Rhizophora can be separated in some 
cases by morphological characters, and an attempt to do 
so will be made in this paper. 
The following description of pollen of several species of 
modern Phizophora is presented after study of the Rhzzo- 
phora pollen available from specimens in the Harvard 
University Herbaria and from the Harvard Pollen Collec- 
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