738 THE SUBDIVISIONS OF THE VEGETABLE KINGDOM. 



Nidus-avis (the Bird's Nest Orchid), and Corallorhiza (the Ooral-root). To this 

 tribe belongs the climbing Vanilla-orchid (Vanilla planifolia) the capsules of 

 which yield the spice. 



To the Vandece and Epidendrece belong the numerous tropical epiphytes of 

 which many genera are widely cultivated in greenhouses, such as Lceliaj Epiden- 

 drum, Gattleya, Masdevallia, Phajus, Gatasetum, (figs. 275 1- ^' '> *■ ^' ^' ''• p. 269), Stan- 

 hopea (Plate XIIL), Phalcenopsis (fig. 258 \ p. 227), Odontoglossum, (Plate XIII.), 

 Dendrohiwm (fig. 275 ^), Oncidium (Plate XIIL), Angrcecum (fig. 418), and many 

 ■others besides (vol. i. Plate III.). 



To give any idea of the enormous varieties of floral structure met with among 

 the Orchids would require a whole volume. In this family of all others do we 

 find adaptations to insect visits carried out on a gigantic scale, and in not a few 

 •cases mechanisms of singular beauty and delicacy. For 'full details Darwin's 

 Fertilization of Orchids should be consulted. 



BurmanniaceoB. — A small family of some 60 species, largely represented in 

 Borneo and New Guinea, is of interest, as it seems to connect the highly specialized 

 Orchidaceae with more typical Monocotyledons like Amaryllidacese. They have 

 •curious flowers, with three or six stamens, and several of them are saprophytic. 



Alliance XXXII. — Fluviales. 



Families: Potamogetacew, Haiadacece, Aponogetacece, Juncaginacece, AlismaceoB, 



ButomacecB, Hydrocharitacece. 



This alliance includes a large number of aquatic forms, some with narrow, some 

 with broad leaves. The gynaeceum is superior, except in the Hydrocharitacese. 

 The stamens and carpels show a tendency to an increased number as compared 

 with typical Monocotyledons. On the other hand, many forms with reduced 

 flowers occur. Endosperm is generally absent. 



Potamogetacece.—lnclnde chiefly submerged forms, some of which raise their 

 inflorescences above the water-level, and are wind-pollinated {Potamogeton, fig. 419), 

 whilst the others, including the Sea-grass (Zostera marina), Zannichellia, &c., are 

 pollinated below the water. Potamogeton (Pondweed) is a large genus of some 

 50 species, met with in fresh and brackish water; Zostera- grows on sandy shores 

 between tide-levels, often forming extensive belts. The embryos in this family are 

 peculiar. They consist of a much -thickened hypocotyl with a relatively small 

 cotyledon inserted upon it. They are termed macropodous. 



There are about 74 species of Potamogetacese. 



Aponogetacece.— Contekins two interesting genera, Aponogeton and Ouvirandra. 

 The plant is submerged, and raises a spicate inflorescence (often forked) above the 

 water. The flowers are imbedded in the spike, and consist of some 6 stamens and 

 3 free carpels, and a small number of perianth-segments. Aponogeton distachus 

 is often cultivated in this country on account of its beautiful white flower-spikes. 

 Ouvirandra fenestralis is the Lattice-leaf plant of Madagascar. 



