HA R D Wl CKE 'S S CIENCE -GOSS IP. 



asked them, give their structural characteristics. It is 

 to such readers of SciENCE-GossiP that we think a 

 short account of them will be acceptable. 



Of course, like all other known organized beings, 

 the Duckweeds are classified and form a distinct 

 family, viz. LemnacecB, the genus Lemna, of which 

 there arc four species found in Britain, being the type 

 of the natural order. Thus we will briefly enumerate 

 the general characteristics of the natural order 

 Lemnacea:. Plants consisting of solitary or clustered 

 green fronds, cellular, or with rudimentary trachea 

 developed, rootless, or witli one or more simple 

 slender roots pi'oceeding from the under parts of the 

 frond, usually tipped by a membranous . sheath ; 

 propagated by budding from marginal clefts in the 

 frond, and by autumnal hybernating bulbils ; very 

 rarely by seed. Flowers most minute, 1-3, contained 

 in a spathe or without a spathe ; floral row absent. 

 Stamens i or 2 ; anther 2 -celled, dehiscing cross- 

 wise ; pollen round, muricate or not ; ovary i -celled ; 

 ovules varying from I to 7, ortholropous, anatropous, 

 or semi-anatropous. Fruit bottle-shaped, not splitting, 

 or splitting transversely. Seeds i or more, with 

 fleshy albumen, or without albumen. 



The Duckweeds are the smallest known flowering 

 plants : they are more or less in all climates, but 

 more especially in temperate regions. They are 

 rarer in the tropics, as the great heat dries up the 

 swamps, and the violent rains greatly agitate the 

 water. They are closely allied to the Aroids by the 

 genus Pistia, which approaches them in the form of in- 

 florescence, and the seed-structure of the genus Grantia 

 closely corresponds with that of Pistia, and the ovule 

 of Lcm7ia trisulca is very similar to the ovule of 

 Orontium. Some scientists give Arum maculatiim 

 the honour of being the progenitor of the Duckweeds ; 

 but let that be as it may, their affinity with the Aroids 

 cannot be doubted. Lindley united them with the 

 Pistias and established the natural order Pistiace?e ; 

 but undoubtedly, as classified by other botanists, 

 the Pistias form a good section of the Aroids, rather 

 than a distinct family. 



The principal genera composing the family are, 

 Lemna,TeImatophace, Spirodela,Wolffia, and Grantia. 

 It would appear that Linneus established the genus 

 Lemna, and included under that category the four 

 species known in Britain. But later on Schleiden 

 established two other genera ; viz. Telmatophace, in 

 which he placed Z. gihha of Linneus, and Spirodela, 

 in which L. polyrhiza of Linneus was placed. Since 

 then, however, the two lattar genera have been made 

 subordinate to the geuus Lemna, which is certainly 

 desirable, as the characters upon which they are 

 founded are insufficient for generic rank. 



Perhaps it will be well to glance at the characters 

 of Lemna proper ; also those of Telmatophace and 

 Spirodela of Schleiden. 



I. Lemna proper. — Root single; ovule solitary, 

 semi-anatropous ; seed horizontal, with a copious 



supply of albumen, — including Z. ;/««w, Linn., and 

 Z. trisulca, Linn. 



2. Telmatophace (Schleiden). — Root single; ovules 

 varying from 2 to 7, anatropous ; seeds erect, with a 

 scanty supply of albumen, or none, — including Z. 

 gibba, Linn. 



3. Spirodela (Schleiden). — Roots numerous ; ovules 

 2, erect, anatropous, — including Z. polyrhiza, Linn. 



It will be easily seen that the above characters may 

 all be included in one genus, making the two latter 

 genera only sub-genera, as Dr. Hooker has already 

 done. 



The characters as he gives them are, " Fronds with 

 one or more simple roots. Flowers in marginal clefts 

 of the fronds. Stamens 1-2. Anthers 2-celled ; 

 pollen muricate. Ovules i to 7." And we think the 

 four British species may be easily disposed of in that 

 way. Those four species we will now briefly describe. 



1. Z. minor, L. (fig. 36). — Frond \-\ inch, ob- 

 ovate or oblong, slightly convex below, green above, 

 paler beneath. Young frond sessile upon the old, 

 soon disconnected. Spathe unequally 2. lipped. 

 Stamens 2. Style moderately long. Distribution 

 almost ubiquitous. 



2. Z. trisulca, L. (fig. 39). — Frond \-\ inch, vary- 

 ing in shape, usually obovate-lanceolate ; tip serrate, 

 or very often entire, proliferous on one or both 

 sides ; young fronds hastate, placed crosswise to 

 the old. Distrib. Europe, Siberia. 



3. Z. [^Telmatophace, Schleiden, fig. yi^S'^'^^i L. — 

 Frond \-\ inch, obovate, or nearly round, opaque, 

 pale green, large air-cells beneath ; young fronds 

 sessile. Stamens 2. Fruit bursting crosswise. Dis- 

 trib. throughout Europe, Siberia, North Africa , 

 America. 



4. Z. [Spirodela, Schleiden, fig. ap) polyrhiza, L. — 

 Frond \-\ inch, broadly obovate, sometimes nearly 

 round, dark green above, purple beneath ; tracheae 

 copious. Spathe 2-lipped. Stamens 2. According 

 to Dr. Hooker, the flower of this species is unknown 

 in Britain. Distrib. throughout Europe, Siberia, 

 North America, &c. 



Besides the genus Lemna we have another genus 

 represented in Britain,— viz. Wolffia ; the characters 

 of which are — "Fronds very minute, rootless, pro- 

 liferous. Flowers bursting through the upper surface 

 of the frond, without a spathe. Anther sessile, 

 i-celled. Ovary globose; style short ; ovule i, erect, 

 orthotropous. Fruit indehiscent. Seed with scanty 

 fleshy albumen. 



There is only one species of Wolffia known in Britain, 

 viz. IV. arrhiza, L., which is the smallest known 

 flowering plant ; the frond being only about ^V i"ch 

 long, and -^ inch broad, loosely cellular beneath. 

 This is found in ponds in Essex, Middlesex, Hants, 

 Surrey, &c. Fig. 38 represents the inflorescence of 

 Wolffia ; fig. 41 represents a section of another plant 

 belonging to Lemnacea; not known in Britain, dis- 

 tinguished from Wolffia by the presence of a root, a 



