686 



BOTANY 



PART II 



Hues of descent, has been taken into consideration in the arrangement of the 

 Monocotylae in relation to the Dicotylae. 



Order 1. Helobiae 



Water or marsh plants with actinomorphic flowers. Gynaeceum, 

 frequently apocarjDous ; carpels in two circles. Seeds, exalbuminous ; 

 embryo, lai'ge. Indehiscent fruits or fruitlets. ■ 



Family 1. Alismaceae (®^). — Widely spread in the warm and temperate zones. 

 Alisma Plantago, Sagittaria sagittifolia, and Butomus umbellatus have long- 

 stalked panicles or iimbels, and occur as marsh-plants. The individual flowers 

 have a calyx and a white (in Butomus, reddish) corolla. Androecium, in BuLomus 

 A 6 4- 3, in Alisma A 6. Gynaeceum apocarpous, in Butomus G3-)-3, in Alisma 

 G 00 . Sagittaria is monoecious with flowers that, by suppression of stamens or 

 carpels, are unisexual. Male flowers, with numerous stamens and sterile carpels ; 



Fif;. 741. — Floral diagram of 

 Echinodorus parmlus, one 

 of the Alismaceae. (After 



ElCHLER.) 



Fig. 7 i2.— Sagittaria sagittifolia. 

 mo\'al of some of the carpels, 

 and Prantl.) 



a, Flower ; b, fruit after re- 

 (Magnilied ; h, after Engler 



female flowei"s, with staminodes and numerous free carpels inserted on the convex 

 floral receptacle (Figs. 741, 742). Leaves in Butomus, linear, channelled, and 

 triangular in cross-section ; in Alisma and Sagittaria, long-stalked with spoon- 

 shaped and sagittate leaf-blades respectively. Individuals of both genera grow- 

 ing in deep flowing water have long ribbon-shaped leaves, similar to those that 

 appear as a transition type in germination ; such plants do not flower. 



Family 2. Juncaginaceae. — Habit grass-like, Triglochin. 



Family 3. Potamogetonaceae.- — Many species of Potamogeton are distributed 

 over the earth in standing or flowing water. Flowers hermaphrodite, tetramerous. 

 Gynaeceum apocarpous. Inflorescence a spike. Leaves usually submerged, with 

 a long sheath, slit on one side, formed from the axillary stipules. P. natans, 

 the common Pond-weed (Fig. 743), at the time of flowering has usually only float- 

 ing leaves, the cylindrical, submerged water-leaves having disappeared by then. 

 liiqjpia maritima and Zanichellia palustris grow in brackish water. Zostcra 

 marina, Grass-wrack, occurs commonly on all north temperate coasts and is used 

 for stuffing cushions (cf. p. 495). 



Family 4. Naiadaceae. — The single genus Naias contains a number of fresh- 

 water plants witli diclinous flowers. It occurs all over the earth. 



Family 5. Hydrocharitaceae. — llydrocharis morsus ranae and Stratiotes aloidcs 

 are floating plants occurring in Britain, which are vegetatively propagated by 



