LEMNACEAE 



501 



(5) There is also a possibility of pollination by small water-spiders, water- 

 beetles, and snails (Planorbis). 



Thus in the pollination of species of Lemnaceae, wind, water, and animals 

 perhaps play equal parts ; cross-pollination without outside help, however, and even 

 autogamy are in no way excluded, as a result of the gregarious habits of plants of 

 the species. 



2938. L. polyrrhiza L. (= Spirodela polyrrhiza Schleid.). (G. Engelman, 

 Bull. Torrey Bot. CI., New York, 1870, pp. 42-3 ; H. Gillman, Amer. Nat., Boston 

 (Mass.), XV, 1 881, pp. 896-7.) — Engelman describes this species as protandrous. 

 Gillman observed that the stamens, which dehisce about 4 p.m., move back at night, 

 and open again the next morning between 7-9 a.m. 



ORDER ALISMACEAE JUSS. 

 ' Bl. u. Insekt. a. d. nordfr. Ins.,' p. 133. 



CXXII. 



Literature. — Knuth, 

 Flowers hermaphrodite or monoecious, rarely dioecious. The inner or both 

 perianth whorls are petaloid, and therefore serve to attract insects. 



957. Alisma L. 



Flowers hermaphrodite and 

 homogamous, white or reddish in 

 colour ; with half-concealed nectar 

 secreted by a ring at the base of 

 the stamens. 



2939. A. Plantago L. 

 (Herm. Miiller, ' Fertlsn.,' p. 565 ; 

 Kirchner, ' Flora v. Stuttgart,' 

 p. I S3; Knuth, 'Bl. u. Insekt. 

 a. d. nordfr. Ins.,' p. 133 ; Warns- 

 torf, Verb. bot. Ver., Berlin, 

 xxxviii, 1896.) — The homoga- 

 mous flowers of this species are 

 arranged in pyramidal panicles, 

 and possess a yellow nectar-guide 

 at the base of each of the three 

 white or reddish petals ; they 

 expand to form a surface about 

 10 mm. in diameter. Hermann 

 Miiller says that nectar is secreted 

 in twelve small drops by the inner 

 side of a fleshy ring formed by the 

 union of the broadened bases of 

 the six filaments. The stamens 

 are directed obliquely upwards and 

 outwards, and turn their pollen- 

 covered surface outwards, while the stigmas stand erect in the middle of the flower. 

 The most frequent visitors are hover-flies, which by flying to the centre effect 



Fig. 415. Alisma Plantago, L, (after Herm. MuUer). (i) 

 Flower seen directly from above. (2) Lateral view of do., after 

 removal of the petals. a, droplets of nectar ; b, stigmas. 



