1882 .] 
413 
[Trelease. 
and first stamen are concerned, this abortion may be considered 
as quite abnormal, due to some local injury or other cause, but 
with the second stamen the case is different. Although it some- 
times equals the first, it is more commonly shorter, even where it 
reaches perfection ; and its very frequent imperfection, taken 
in connection with this fact, is suggestive when Lemna is com- 
pared with the monandrous Wolffia. 
Saving the yellow pollen, there is no colored part in the flower. 
The pollen grains are spherical, studded with very minute points, 
by the mutual action of which the grains remain heaped up in the 
open anther after dehiscence. 
The origin of the color and roughness of the grains admit of 
several conjectures. They may be useful acquisitions connected 
with the pollination of the flowers, or they may have been inherited 
from ancestors to which they were of value. If the latter sup- 
position be true so simple a plant as Lemna must have received 
these specialized features from more highly organized ancestors, 
which, according to Agardh, would be some of the Araceae. If, 
however, they are valuable acquisitions whose preservation and 
augmentation are due to natural selection, they may have been de- 
rived from ancestors much inferior to the Aroids. The color of the 
pollen does not appear to be of any use to the plant ; on the other 
hand, the roughness of the grains is of immediate use, in ena- 
bling the mass to resist slight jolts to which the plants are liable 
from their floating habit, and it may possibly aid in attaching the 
pollen to the stigma when it is brought in passing contact with 
this organ. Had both been derived from more highly organized 
ancestors, the roughness would be preserved because of its con- 
tinued value ; and the color might perhaps be preserved from 
obscure causes such as are associated with the hitherto unex- 
plained colors of several floral and other organs. 
By its structure and habit Lemna is admirably fitted for cross- 
fertilization by the aid of currents of water — a hydrophilous 
plant. Surface currents due to the wind or other causes crowd 
the plantlets together m masses m which the relations of the sev- 
eral individuals are incessantly changing. In this way the recep- 
tive stigmas of young flowers must be repeatedly pushed against 
the anthers of those that are older, by whose pollen they are fer- 
