1882 .] 
415 
[Trelease. 
at noon or in the afternoon.” Learning, later, that the specimens 
on which the preceding note was written had been allowed to 
bloom indoors, Dr. Englemann suggests that lack of vitality may 
be the reason for the undeveloped condition of the second stamen, 
and adds : “ In Lemnae collected out of doors, ‘ wild,’ I find some- 
times one, sometimes the other fully developed ; and, if the sec- 
ond or posterior one is just opening, the other is always effete 
and elongated, a condition which I have never seen in this Spiro- 
dela. After shedding pollen the second also elongates, as in water 
Callitriches ” (2, 10-11). It may be, therefore, that in Spirodela 
the order of maturation is different from what we find in Lemna. 
In speaking of S. polyrrhiza Mr. Gilman (1, 897) says : “ At 4h. 
15m. p. m. both stamens were expanded ; but closed at night, when 
I placed them in a vessel of water. At 7 o’clock next morning 
they had commenced to expand, three sets of flowers being in 
that condition. At 7h. 45m. a. m. eighteen had expanded, while 
by 8h. 30m. over thirty sets of flowers had opened, remaining so 
all day. They invariably closed towards night. In each instance 
one of the two stamens was always in advance of the other in 
ripening its pollen.” The reported absence of the flowers in the 
morning, their gradual appearance during the day, and their dis- 
appearance at night, taken in connection with Dr. Engelmann’s 
statement, show that Spirodela is worthy of careful study by those 
who are fortunate enough to obtain living material. The nature of 
the phenomena noted by Mr. Gilman must, apparently, be different 
from the opening and closing of flowers having a perianth, for in 
so simple a flower as that of Spirodela such a process is not conceiv- 
able. 
In all probability those who have the different species in flower 
will find that the process of maturation in all is quite similar to 
what has been described for L. minor, and affords nearly or 
quite equal facilities for crossing. In Wolffia, where the flower 
is reduced to a single stamen and pistil seated in a cavity in the 
frond, I should expect protogyny ; and crossing might readily be 
effected as these minute plants roll and brush by one another 
under the influence of surface currents. 
