220 WOLFFIA ARRHIZA IN ENGLAND. 
* Commentatio de Lemna,’ Altdorff, 1801) was founded by Horkel on 
an Egyptian plant to which I shall presently allude, and was first ac- 
curately defined by Schleiden in 1839 (Linnza, vol. xiii. p. 389). In 
this paper only one species, W. Delilii, is given, but in a reprint of 
the paper in 1844 (Beitrüge zur Botanik, vol. i. p. 233), the author 
added the Lemna arrhiza of Linnzus to the genus, under the name of 
T. Michelii. Yn 1849, Weddell (Aun. des Sc. Nat., 3rd series, vol. 
xii. p. 170) described another species, W. Brasiliensis. 
From these papers, and an elaborate memoir on Lemna arrhiza 
(L.), by Hoffmann, published in 1840, in Weigmann's Archiv., and 
translated in the Ann. des Sc. Nat., 2nd ser., vol. xiv. p. 223, the fol- 
lowing definition of the genus has been derived :— 
Worrria, Horkel and Sehleiden.— Horkelia, Rchb.—Fiowers mone- 
cious, altogether naked. Male flower :—Stamen 1. Filament very 
short and thick; anther subglobose, unilocular. Female flower :— 
Ovary one, 1-celled, 1-seeded. Ovule atropous, nearly erect Style 
very short. Fruit a spherical, l-seeded, indehiscent utricle. 
globose, erect, with a double integument, the outer fleshy, the inner 
thinner, indurated above, and forming an operculum pervious at the 
apex. Embryo thick, turbinate, in the axis of the seed, radicle 
superior.— Flowers arising from a pit in the centre of the upper 
surface of the frond. Plants increasing chiefly by gemmation, the 
bud (new frond) single, growing from within the base of the ex- 
tremity of the parent frond and shortly stalked. Rootless. No 
spiral vessels. 
The plants composing this genus seem to require separation from 
Lemna on account of their different mode of gemmation, the absence 
of roots, the central position of the flowers, and the single stamen. 
The cells composing the epidermis are also of a different shape, being 
bounded by straight instead of flexuose sides, as in Lemna, the stomata 
also are larger and the cellular structure of the frond more lax than in 
the species of the latter genus. 
The synonymy of the British species is as follows :— 
Lenticularia omnium minima, arrhiza, Micheli, Nov. Plant. Genera, 
p. 16 (1729). 
Lemna arrhiza, Linn. Mant. ii. 294 (1767). . 
Wolfia Michelii, Schleiden, Beitr. z. Botan. i. p. 233 (1844). 
“ Telmatophace arrhiza," Welwitsch, Herb. Lusitan. (sp. coll. 1848). 
