60 THE AMERICAN NATURALIST. [VoL. XXXII. 
gain in worth in proportion as their originality begins at a higher 
plane and is built upon a surer foundation than could have been the 
case in other groups. 
To this suite of useful papers Mr.C. H. Thompson has just added a 
revision of the North American Lemnaceæ. These diminutive aqua- 
tics, popularly called duck meats, include the most minute flowering 
plants. While from their peculiar structure they have long been 
familiar examples of such morphological phenomena as _ phylloidal 
stem, vegetative reproduction, reduction of floral structures, etc., their 
systematic interrelationship and geographic distribution have been, 
notwithstanding the critical treatises of Hegelmaier and Engelmann, 
but imperfectly understood. Mr. Thompson’s paper is the first upon 
its peculiar field, since no previous monograph has at once covered 
and been restricted to North America. 
While Engler in the Matirlichen Pflanzenfamilien reduces Wolffi- 
ella to a subgenus of Wolffia, Mr. Thompson follows Hegelmaier in 
recognizing four genera in the family, namely, Spirodela, Lemna, 
Wolffia, and Wolffiella, but rearranges them so that Wolffiella may 
stand next Lemna. No change is made in the North American 
Spirodela (represented by the common S. polyrrhiza), but a new 
South American species of somewhat doubtful identity and remark- 
ably dissevered range is added to the genus. In Lemna the recog- 
nized North American species are Z. gibba, minor, trisulca, perpusilla 
(with var. ¢rinervis), cyclostasa, and minima. By the name ZL. cyclostasa 
(Ell.) Chev. is designated the plant which has for some years been 
known as Z. valdiviana Phil., since the latter species, as the author 
believes, is identical with the Z. minor var.? cyclostasa of Elliott’s 
Botany of South Carolina and Georgia. It is a pleasure to see that 
the range of this species, unaccountably incomplete in Britton and 
Brown’s Zora, is duly extended to the three southern New England 
states. In Wolffiella three North American species are recognized ; 
namely, W. floridana (Wolfia gladiata, var. floridana J. D. Smith), 
W. oblonga, and W. lingulata. Of Wolffa there are also three species 
credited to the continent, — W. papulifera (a new species from Mis- 
souri, discovered by Bush), W. punctata, and W. columbiana. 
Mr. Thompson’s descriptions are clear and ample, and the copious 
outline illustrations, which are of his own drawing, are satisfactory. 
His observations upon the “resting stages” (Hegelmaier’s Winter- 
sprosse) are worthy of mention, and above all the careful citation of 
synonymy and enumeration of exsiccati make the paper a very 
welcome contribution to American systematic botany. B. L. R. 
