

Oh. XIII] THE MONOCOTYLEDONS 527 



Ecologically the Angiosperms are typical terrestrial meso- 

 phytes, though extending into many other situations. They 

 are at present the dominating and expanding group, still in 

 active evolution. Phylogenetically their relations with the 

 Gymnosperms have been variously explained in the past, 

 but the present evidence seems rather clearly to demon- 

 strate their origin from an extinct group called Bennettitales 

 which in turn was a branch of the Cycadofilicales (Fig. 

 275). Thus no direct connection would exist between the 

 present Gymnosperms and Angiosperms, though they are 

 related through a not too distant ancestry. 



The Angiosperms fall into two sharply-marked sub-classes, 

 as follows : 



Sub-class 1. Monocotyledone.e : the Monocotyle- 

 dons or Endogens. 



Sub-class 2. Dicotyledone^e : the Dicotyledons, or 

 Exogens. 



Sub-class 1. Monocotyledone^ : the Monocotyle- 

 dons, or Endogens. 



These are distinguished from the Dicotyledons by the 

 presence of one cotyledon instead of two in the embryo, by 

 a usually parallel instead of netted venation of the leaves, by 

 prevalence of the number 3 instead of 4 or 5 in the whorls of 

 the flower, and by endogenous instead of exogenous struc- 

 ture of the stem (page 127). The fibro- vascular bundles are 

 separate, closed (without cambium), and scattered in a 

 general ground tissue. They include about 24,000 of the 

 133,000 known Angiosperms, and are most abundant towards 

 the tropics. 



As a whole they are inclined to herbaceous texture, only 

 a few tropical kinds, especially Palms, becoming woody and 

 forming real trees. They prefer humid situations, and 

 hence prevail in damp forests and meadows, while many 

 have taken to life in the water as true hydrophytes; but 

 others live also in dry places, even in deserts. The closed 



