HazEN: TRIMORPHISM AND INSECT VISITORS OF PONTEDERIA 477 
ments of each of the two similar sets of grains given above were 
combined, and the mean of the three diameters of the average 
grain of each kind was then taken; this makes the mean diameter 
for all the large grains 43.3 microns, for all the mid-size grains 
34.66 microns, and for all the small grains 22.16 microns. Оп this 
basis the ratio of all the large to all the mid-size grains is as 100 
to 80, and the ratio of the latge to all the small grains is as 100 
to 51, which is a slightly greater difference than that reported by 
Halsted, who pointed out the fact that Pontederia cordata shows 
the greatest range of pollen size yet recorded for any flower. 
This method of averaging, however, is not accurate, and in any 
case comparison of volumes would seem to be more significant. 
Computing the volume of spheroids with diameters represented 
by the measurements detailed above, or, more simply, calculating 
the ratios only by use of logarithmic tables, it is found that the 
volume average of the two sets of large grains is to that of the mid- 
sized grains as 100 is to 53, and Шетайо of the volume of the large 
to that of the small is approximately as 100 to 14. It will be 
seen that these ratios present a much better basis for comparison 
with the ratios of style length than the ratios of the diameters. 
Darwin's comparison, however, was based on the extremes of size 
in single sets of pollen grains. In our plant the largest and smallest 
sets of grains are found in the mid-styled form, where the ratio of 
mean diameters is as 100 to 50, and the volume ratio about as 
100 to 13. In our species, as in Darwin's Brazilian plant, the 
pollen grains of both sets of stamens in the short-styled flower are 
slightly smaller than those of the stamens of corresponding length 
in the other flower forms. Я 
The significance ої these differences in pollen size is a point 
of much interest. Delpino regarded the difference in size as a 
direct adaptation to the style length, supposing the larger grains 
could produce a pollen-tube long enough to penetrate the length 
of the long style, and that the tube of the smallest grains would 
readily grow only the length of the short style. This view Fritz 
Müller considered confirmed by his experimental work with Eich- 
hornia crassipes* where he found that long- and mid-styled pistils 
* Müller found, for example, that flowers on a long-styled spike legitimately 
fertilized by pollen from the long stamens of mid-styled flowers produced 141.7 
