SECT. II 



PHANEROGAMIA 



685 



two-ranked or alternate arrangement the leaves, which have long 

 sheaths and continue to grow for a considerable time at their bases". 

 The growth of the stem is often limited ; branching is in many cases 

 entirely wanting, and rarely results in the development of a highly 

 branched shoot-system. The leaves are mostly sessile and parallel- 

 veined, and of a narrow, elongated, linear, or elliptical shape 

 (Fig. 739). _ 



Anatomically the Monocotyledons 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. 137), 



Secondary thickening is conse- 

 quently wanting in Monocotyle- 

 dons, and in the rare cases in 

 which it is found results from 

 the formation at the periphery 

 of the central cylinder of ad- 

 ditional closed bundles embedded 

 in ground-tissue (cf. p. 155). 



9 



Fin. 739. — Ijeat of rulyrjonatnin viultiflorum 

 with parallel venation. (| nat. size.) 



Pig. 740. — Diagram of a typical Monocoty- 

 lodonous flower. 



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. 740). The 

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

 number of A\horls, or of the members within a whorl, is only found in 

 the Helobiae. 



In this feature and in the apocarpous gynaeceum of the 

 Polycarpicae, among the Dicotyledons, can be recognised 

 made to derive tlie Monocotyledons from the polycarpic 

 this order {**'). This assumption is further supported 

 l^) of the carpels, and apparently r f the stamens also 

 Tliis possible origin of the Monocotyledons, since no otl 

 and the Polycarpicae have formed the point of departur 



Helobiae an approach to the 

 , and the attempt has been 

 Dicotyledons by means of 

 by the spiral arrangement 

 , in the genus Sagittaria. 

 ler has yet been indicated, 

 e of a number of divergent 



