HYDROCHARIDEAE 393 



815. Stratiotes L. 



Flowers dioecious ; white ; with half-concealed nectar. 



2602. S. aloides L. (Sprengel, 'Entd. Geh.,' pp. 441-2; Nolle, ' Bot. 

 Bemerk.'; MacLeod, Bot. Jaarb. Dodonaea, Ghent, v, 1893, p. 286; Ascherson, 

 Verh. bot. Ver., Berlin, xvii, 1875, pp. 80-5; Knuth, ' Bloemenbiol. Bijdragen.') — 

 MacLeod states that the male flowers of this species possess about 1 2 fertile stamens 

 and 15-30 sterile ones; the latter serve as nectaries and are situated between the 

 ordinary stamens and the petals. The nectaries of the female flowers are like those 

 of the male ones. In many districts, e. g. Scandinavia and Denmark, only the female 

 plant occurs ; this seems also to be the case in North Schleswig, while in Holstein the 

 male plant is not rare. The nectaries in both flowers consist of numerous bright 

 yellow, glandular threads. Nolte says that fruit- and seed-formation take place 

 even without fertilization, and prolific vegetative propagation is effected from the 

 rhizome. 



Visitors. — Knuth observed the hover-fly Eristalis tenax L. 



816. Hydrocharis L. 



Flowers dioecious; with half-concealed nectar. 



2603. H. Morsus-ranae L. (Delpino, 'Ult. oss.,' II; Ascherson u. Gurke, 

 op. cit. ; MacLeod, Bot. Jaarb. Dodonaea, Ghent, v, 1893, pp. 285-6; Knuth, 

 'Bloemenbiol. Bijdragen'; Warnstorf, Verh. bot. Ver., Berlin, xxxviii, 1896.) — The 

 white petals in this species possess a nectar-scale on the inner surface of their bases. 

 The flowers of each kind show vestiges of the opposite sex. Warnstorf says that the 

 yellow stamens possess broad papillose filaments; the anthers dehisce by a lateral 

 slit. The pollen-grains are yellow in colour, rounded tetrahedral, beset with spines 

 causing them to cling to one another and to the walls of the anthers for a long time. 

 The stigmas of the female flowers are yellow in colour, grooved internally, or forked 

 and closely beset with long papillae. 



Visitors. — Knuth observed numerous honey-bees, skg. 



CVII. ORDER ORCHIDEAE JUSS. 



Literature. — Darwin, 'Orchids'; Herm. Muller, 'Fertilisation,' pp. 527-41; 

 Pfilzer, vide Bibliography, Vol. I, p. 330, Nos. 2782-4; Knuth, ' Grundriss d. 

 Blutenbiol.,' pp. 95-6 ; Kirchner, ' Flora v. Stuttgart,' p. 163. 



Orchids are distinguished by a greater variety of flower-forms than any other 

 order of plants. These forms are adapted in such a remarkable way for cross- 

 pollination by insects that the structure of the flower corresponds in its smallest 

 details to peculiarities of their bodies. Automatic self-pollination therefore only 

 occurs as an exception {f.g. Ophrys apifera Huds. is self-fertile, according to 

 Darwin), and is much more generally excluded by the relative positions of stigmas 

 and anthers ; but these opposite conditions are united by an uninterrupted chain of 

 transitions, as the following list, drawn up by Hermann Muller, shows : 



I. Cleistogamous flowers: — Schomburgkia, Cattleya, Epidendrum (H. Criiger), 

 Dendrobium (Anderson). 



