DIV. II 



ANGIOSPERMAE 



719 



(Fig. 791). The pinnate or palmate leaves of the Palms and the 

 perforated leaves of some Araceae are due to the perishing of definite 



portions of the lamina during de- 

 velopment. 



Anatomically the Monocotyle- 

 dons are characterised by their 

 closed vascular bundles in which 

 no cambium is developed ; these 

 are uniformly scattered in the cross- 

 section of the stem (cf. Fig. 109). 



FIG. 791. Leaf of Polygonal urn multiflomm FIG. 792. Diagram of a typical Mono- 



with parallel venation (f nat. size). cotyledonous flower. 



Secondary thickening is consequently wanting in Monocotyledons, 

 and in the rare cases in which it is found results from the formation 

 at the periphery of the central cylinder of additional closed bundles 

 embedded in ground-tissue (cf. p. 142). 



The flower in the Monocotyledons is usually pentacyclic and has 

 two whorls constituting the perianth, an androecium of two whorls, 

 and a gynaeceum of a single whorl. The typical number of members 

 in each whorl is three. The two whorls of the perianth are usually 

 similarly formed and thus constitute a perigone (Fig. 792). The 

 floral formula of such a flower is P 3 + 3, A 3 + 3, G (3). 



(a) Flowers adinomorphic 

 Order 1. Helobiae 



This order includes only aquatic or marsh plants. The radial or 

 actinomorphic flowers have the gynaeceum frequently apocarpous and 

 composed of two whorls of carpels, which develop into indehiscent 

 fruitlets or follicles. Seeds exalbuminous ; embryo large. The order 

 connects by its floral structure the Monocotyledons with the Poly- 

 carpicae (cf. p. 630) ( 14 ). 



Family 1. Alismaceae. Widely spread in the warm and temperate zones. 

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



