522 



BOTANY 



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. 513). 



Anatomically the Monocotyledons are characterised by their 

 closed vascular bundles in which no cambium 

 is developed ; these are uniformly scattered iu 

 the cross section of the stem (cf. Fig. 101). 

 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. 144). 



The flower in the Monocotyledons is usually 

 pentacyclic and has two g 



whorls constituting the peri- 

 anth, 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. 5 1 4). The floral formula 

 of such a flower is P 3 + 3, 

 A 3 + 3, G (3). An increased number of whorls, or of the members 

 within a whorl, is only found in the Helobiae. 



Iu this and in the apocarpous" gynaeceum of the Helobiae an approach to the 

 Polycarpicae, among the Dicotyledons, can be recognised, and the attempt has been 

 made to derive the Monocotyledons from the polycarpic Dicotyledons by means of 

 this order. This assumption is further supported by the spiral arrangenieht ( !:til ) of 

 the carpels, and apparently of the stamen* also, in the genus Sa/jittui-iu. This 

 possible origin of the Monocotyledons deserves more thorough investigation since no 

 other has yet been indicated, and the Polycarpicae have formed the point of 

 departure of a number of divergent lines of descent. 



Kit;. ">!;!.- I .i-,-it'o !'/'<' 



multijliiriiiii with piinillrl 

 venation. (:} nut. six*-.) 



;. 514. Diagram of u 



typical Monocotyle- 

 donous llower. 



Order 1. Helobiae 



Water or marsh plants with actinomorphic flowers. Gynaeceum, 

 frequently apocarpous ; carpels in two circles. Seeds, exalbuminou's ; 

 embryo, large. Indehiscent fruits. 



