Dimorphic Anthers of Lager stroemia indie a. 505 
Robertson, whose work on floral ecology and on bees is well known, were 
taken : 
Megachile mendica , Cr. $ 
Megachile generosa , Cr. $ 
Bombus scutellaris , Cr. (B. f rater nus, Sm.) £ 
Bombus americanorum , Cr. (j5. pennsylv aniens > De G.) £ 
Chloralictus versatus , Rob., seems to visit the larger stamens frequently, 
but was also observed on the smaller stamens. 
Apis mellifera , L. g 
Agapostemon radiatus , Say. ? 
VI. Summary and Discussion of Observations. 
In the Malayan shrub. Lagerstroemia indica , the stamens are dimorphic, 
those of the outer whorl being larger than the more central yellow group, 
and differing from them not only in colour, but, as Darwin showed, in the 
pollen which they contain. 
Quantitatively, the two types of anthers bear the size (weight) ratio of 
about i*3 : 1. The filaments of the large* anthers are about 5-6 times as 
large (heavy) as those of the small ones. 
The observations described in this paper show that the differentiation 
is not merely morphological, but that it is physiological as well. The 
larger (outer) anthers lose water much more rapidly by evaporation than 
do the smaller ones. The total loss is finally sensibly the same in the 
two types. 
The underlying cause of the physiological differentiation seems to be 
chemical rather than physical. The large anthers present, in comparison 
with their volume, a smaller surface from which evaporation can take place, 
and should in consequence be expected to show a lower, instead of a higher 
initial water loss. Furthermore, there is no conspicuous difference in the 
histology of the superficial layers of cells of the two types of anthers. On 
the other hand, there appears to be a water-soluble substance occurring 
only, or in greater abundance, in the smaller anthers which lowers their rate 
of water loss. The nature of this substance is quite unknown. 
The ecological consequence 1 of the physiological differentiation is 
that the pollen of the larger anthers (which are borne on longer filaments, 
incurved in a manner to bring them in contact with the body of a visiting 
insect) is dry and powdery soon after the opening of the flower, and easily 
scattered over the body of a visiting insect, whereas that of the smaller 
anthers remains for a much longer time moist, and easily massed by the 
bees, which collect it in large quantities. 
1 I have reviewed elsewhere (loc. cit.) the literature of the biology of the flower of this form. 
It seems unnecessary to go over once more the points in detail. 
