498 ANGIOSPERMAE—MONOCOTYLEDONES 



as occasional visitors ; they creep over the crowded anthers and stigmas forming 

 a continuous surface, and easily transfer pollen to the latter. 



955. Acorus L. 



Protogynous, sessile, hermaphrodite flowers, arranged on a spheroido-cylindrical 

 spadix. 



2933. A. Calamus L. (Kerner, 'Nat. Hist. PI./ Eng. Ed. i, II, p. 402; 

 Ludwig, ' Zur Biol. d. phanerog. Siisswasserflora,' p. 128; Knuth, ' Bl. u. Insekt. 

 a. d. nordfr. Ins.,' pp. 139-40.) — The spadix of this species, up to 10 cm. long and 

 i^ cm. thick, is completely covered by several hundred (700-800) closely-crowded 

 flowers, each possessing a sessile, punctiform stigma and six stamens, 12 mm. long. 

 Formation of the berry-like fruits has never been observed in Europe, though they set 

 in Japan and India. Ludwig ascribes this to the circumstance that all European 

 Calamus plants are supposed to have been derived from a single stock, introduced 

 by Clusius. According to this, the species is adynamandrous. 



This attempted explanation seems to me more probable than the one given by 

 Kerner, who says that the species of Calamus set no fruit in Europe because the 

 insects which poUinate them are absent. Autogamy is completely excluded by marked 

 protogyny ; and Kerner says that geitonogamy cannot take place automatically by 

 fall of pollen, this being adherent, but can only be effected by the help of insects. 

 Judging from the construction of the spadix, all our native Diptera and Hymenoptera 

 would be able to transfer pollen ; but insect-visits have not yet been observed in 

 Europe. Warnstorf describes the pollen-grains as yellowish in colour, very small, 

 ellipsoidal to oval, glabrous; about 12 /j, broad and 18-22 /j long. {C/. Loew, 

 ' Blutenbiol. Floristik,' p. 363.) 



CXXI. ORDER LEMNACEAE LINK. 

 956. Lemna L. 



Literature. — Ludwig, ' Siisswasserflora,' pp. 38-40 ; Trelease, Proc. Soc. Nat. 

 Hist., Boston (Mass.), xxi, 1882, pp. 410-15; Hegelmaier, ' Lemnaceae,' in Engler 

 u. Prantl's ' D. nat. Pflanzenfam.,' II, 3 ; Delpino, Riv. bot. dell' anno 1880 ; Knuth, 

 ' Bl. u. Insekt. a. d. nordfr. Ins.,' p. 138 ; Warming, Verh. bot. Ver., xxxviii, 1896. 



2934. L. arrhiza L. (= Wolfiia Michelii Schleid.); 2935. L. trisulca L. ; 

 2936. L. minor L. ; and 2937. L. gibba L. — Hermaphrodite or monoecious 

 species, seldom flowering in Germany. Propagation is therefore almost exclusively 

 effected by the sprouting of the thalloid, usually lens-shaped, floating stem. L. arrhiza 

 does not flower at all in Germany, but only in warmer regions. The accounts of 

 the flower mechanism given by different investigators are contradictory in part, but 

 may, in Ludwig's opinion, all be correct, as the mechanism of the same plant may 

 vary in different regions. 



As I have never had an opportunity of observing species of Lemna in flower, 

 I will give Ludwig's description of the oecology of L. minor, as he observed them in 

 a room, and in a sheltered pond in the neighbourhood of Greiz from May to July. 

 The monoecious inflorescence consists either of one short-styled pistil above and two 



