Trelease.] 
414 
[March 15, 
tilized; While crossing is well provided for, self-fertilization is 
not entirely excluded. As the stigma, if unfertilized, remains 
fresh until after the first stamen matures, pollen may and occa- 
sionally does fall from the higher anther upon it, effecting fertili- 
zation. Here, as is usual, the first chance is for crossing. 
So far as I know, the few writers on Lemna who touch upon 
pollination hold to the idea of close-fertilization as the rule. Heg- 
elmaier (1, 108-9) remarks on the protogyny of L. minor; but 
found L. gibba to be partly protandrous, the first stamen emerg- 
ing before the maturation of the pistil. So far as appearances go, 
he remarks, the stigma, if unfertilized, remains receptive until 
the last stamen has matured, adding : “ Lemna is so organized that 
the pistil can be fertilized by the pollen of its own stamens, and 
this can scarcely fail to occur in favorable weather. The stigma 
is usually covered with pollen which from the relative positions of 
the parts must have come from one or the other anther, while a 
transfer from other plants, so far as can be seen, is very improba- 
ble and does not appear favored by any of the usual auxiliaries.” 
Mr. Roper (1, 29), speaking more particularly of L. gibba, says 
that the lack of synchronism between the stamens is “provided 
simply for the preservation of the species ; the plant floating on 
the surface of the water, and, being thus exposed to every vicissi- 
tude of weather, is so planned that should wind or rain disperse 
the pollen from one stamen, the ovule may be fertilized by the 
other when it becomes mature.” 
As to other species, Hegelmaier’s figures show that the stamens 
mature in the order described for L. minor in L. trisulca (1, pi. 5, 
f. 18), L. valdiviana 1 (pi. 7, f. 2), and L. paucicostata (pi. 8, f. 2) ; 
and the same development is mentioned by him for Spirodela poly- 
rrhiza (2, 627). Dr. Engelmann (1, 42), writing of the latter, 
says : “ I find the singular circumstance of the first (anterior) sta- 
men only being developed and protruding (but never so far as in 
the other species), and the second or posterior one and the pistil 
being enclosed. Is this owing to the hour of the day when col- 
lected ? It would be worth while to find out, whether in Lemna 
the first stamen may be developed in the morning, and the second 
1 L. Torreyi Austin, of the Manual. 
