HELOBIE33. 



281 



place under water. Posidonia and Cymodocea are allied to these. About 

 70 species. 



Order 3. Aponogetonaceae. Aquatic plants with tuberous stem. They have 

 a single, petaloid perianth (3-2-1 -leaved), most frequently 6 stamens and 3(-6) 

 carpe-s. Straight embryo. About 15 species (Africa, Madagascar, Tropical 

 Asia and Australia). Aponogeton distachyus and A. (Ouvirandra) fenestralis are 

 grown in conservatories ; the latter has lattice-like, perforated leaves. 



Order 4. Najadacese. Only one genus Najas (about 10 species) ; annual 

 fresh water plants with leaves in pairs and solitary, unisexual flowers. The c? 

 flower is remarkable in having a terminal stamen, which has either 4 longi- 

 tudinal loculi or 1 central one ; on this account the stamen of Nuias is con- 

 sidered by some authorities to be a stem and not a leaf-structure. The uni- 

 locular gynoeceum and the single, erect, anatropous ovule are also terminal. 

 Pollination takes place under the water. 



Order 5. Alisrnacese. The regular, hypogynous flowers are in 

 some species unisexual by the suppression of either androecium or 

 gynoeceum ; they have a 6-merous perianth, generally differentiated 

 into 3 .epals and 3 petals ; generally 6 stamens in the outer whorl 

 (by the division of the 3 ; Fig. 282) and 

 often several 3-merous whorls inside 

 these, and 6-00 free carpels arranged 

 cyclically or spirally. Fruit a syncarp. 

 Marsh- or water-plants with radical 

 leaves and long-stalked inflorescences. 



A. Butomece. Follicles with many 

 seeds, u'hich are borne on nearly the 

 whole of the inner surface of the cyclic 

 carpels (as in Nymph aeacese). Embryo 

 straight. Butomus (Flowering Rush, 



Fig. 282), has an umbel (generally composed of 3 helicoid cymes). 

 S3, P3, stamens 9 (6 + 3, i.e. the outer whorl doubled), G 3 + 3. 

 B. umbellatus ; creeping rhizome with triangular Iris-like leaves. Hydrocleis. 

 Limnocharis. 



B. Alismece. Fruit achenes. Latex common (in the inter- 

 cellular spaces). The flowers are arranged most frequently in 

 single or compound whorls. Embryo curved, horse-shoe shaped. 

 Alisma has S 3, P 3, A 6 (in 1 whorl, grouped in pairs, i.e. doubled 

 in front of the sepals), and 1 ivhorl of 1-seeded achenes on a flat 

 receptacle. The leaves are most frequently radicle, long-stalked; 

 the lamina have curved longitudinal veins, and a richly branched 

 venation. A. plantugo. Elisma (E. natans) has epitropous (turned 

 inwards) ovules, whilst the ovules of Alisma, Sagittaria and others 

 are apotropous (turned outwards). Echinodorus (E. ranunculoides) 



FIG. 282. Diagram of Butomus : 

 /bracteole. 



