124 



PISTIACEjE. 



[ E.ndouens. 



ORDER XXXIV. PISTIACE, ; E.--Lemnads, or Duckweeds. 



I'istiaceje, Rich, in Humb. et Bonpl. N. G. et Sp. 1. 81. (1815) ; Lindl. in Hookers Fl. Scot. 2. 191. 

 (1821); Svnops.2bl. (1829); E 'mil. Gen. p. 233 ; Meisner., p. 363; Kunth enum.3 7; BUtmc, 

 Rumphia'2. 76.— Lemnaceje, DC. and Duby, 532. (1828); Endl. Gen. p. 232; M eitner, p. 363 ; 

 Kuntli. enum. 3. 2. Schkiden in Linncea, xiii. 384 ; Hoffman in Tydschi: v. nat. Gcxch. 

 Leyden (1838). 



Diagnosis. — Aral Eiidogens with 2 or 3 flowers, of which one only is ? , no spadix, a 

 one-celled ovary, erect ovules, and a slit embryo. 



Floating or land plants, with very cellular, lenticular, or lobed fronds or leaves, some 

 of them wholly destitute of spiral vessels, except perhaps in the pistil. Flowers appear- 

 ing from the margin of the fronds, 2 or 3, naked, enclosed in 

 a spathe, but without a spadix. $ Stamens definite, often mona- 

 delphous (pollen globose, muricated, with a single aperture in Lemna 

 Schleiden) : $ Ovary 1 -celled, with one or more erect ovules ; style 

 short ; stigma simple ; ovules anatropal, hemianatropal, or atro- 

 pal. Fruit membranous or capsular, not opening, 1- or more- 

 seeded. Seeds with a fungous testa, and a thickened indurated 

 foramen ; embryo either hi the axis of fleshy albumen, and having 

 a lateral cleft for the emission of the plumule, or at the apex of the 

 nucleus, covered in by a hardened endostome. 



The common Duckweed (Lemna) may be regarded as being the 

 most simple of all Phamogamous plants. Its stem and leaves are 

 fused into a minute lenticular frond, which pullulates by openings 

 in its sides ; its roots are simple fibres, tipped by a calyptra, which 

 Schleiden regards as a peculiar organ, and its flowers are two in 

 number, one male and the other female, lying concealed in a slit of 

 the frond ; they have neither calyx nor corolla, but are enclosed in 

 a delicate membranous bag. Lemna is indeed but one remove 

 from a Crystalwort (Riccia, p. 57) ; species of which have even 

 been mistaken for Lemnas by some authors, according to Schleiden. 



All the true Lemnas are almost entirely destitute of spiral 

 vessels, which the same author found abundantly in the old 

 L. polyrhiza, now called Spirodela. A Lemna indeed may be 

 said to consist of a small plate of cellular tissue, and a couple or 

 three flowers. There is however in the fresh water of tropical 

 countries a very common floating plant, called Pistia, which may be 

 regarded as a Lemna with the leaves and stem separated, and the 

 flowers more highly developed ; there being a distinct spathe for the 

 inflorescence, and a kind of cup-like calyx to the male flower. And 

 then again the Mediterranean gives birth to Ambrosinia, a little 

 land plant, with leaves of an ordinary kind, and a small spathe in- 

 closing a couple of flowers, of which the uppermost has many mona- 

 delphous stamens, perfectly destitute of a calyx, and an ovary which 

 is like that of Pistia. If we disregard the simplicity of this structure, 

 and consider the organisation as if it belonged to plants of a more 

 highly developed character, it will be found that these are really 

 nothing but Arads, the spadix of which is reduced to two or three 

 flowers of different sexes. But while the accuracy of this view of 

 the nature of the Duckweed order is generally acknowledged, it must 

 be borne in mind that this very reduction of parts is inconsistent 

 with the notion of Arads, properly so called ; and hence the necessity of constituting 

 a particular order. I find from an examination of seeds of Pistia, most kindly procured 

 from India for me by Dr. Wallich, that the embryo is a minute body lying within the 

 apex of the albumen ; in Lemna it occupies the axis ; in both there is a fungous testa, 

 a remarkable induration of the foramen of the secundine. The embryo of Pistia is 



I 



LXXXlll. 



with 



very minute, and, as far as I can see, solid ; but Horkel says it is slit, and in Lemna there 

 is certainly a cleft on one side for the emission of the plumule, just as in Arads. Most 



Fig. I.X XX III.— Pistia Stratiotes. 

 seed; 4. the same cut perpendicularly. 



A spathe with its 2 flowers ; 2. a section of a seed-vessel ; 3. a 



