CHAPTER XXV 



MONOCOTYLEDONS AND DICOTYLEDONS 



249. Contrasting characters. — The two great groups of 

 Angiosperms are quite distinct, and there is usually no dif- 

 ficulty in recognizing them. The monocotyledons are 

 usually regarded as the older and the simpler forms, and 

 are represented by about twenty thousand species. The 

 Dicotyledons are much more abundant and diversified, con- 

 taining about eighty thousand species, and form the domi- 

 nant vegetation almost everywhere. 

 The chief contrasting characters 

 may be stated as follows : 



Mo7iocotyledo7is. — (1) Embryo 

 with terminal cotyledon and lat- 

 eral stem-tip. This character is 

 practically without exception. 



(2) Vascular bundles of stem 

 scattered (Fig. 332). This means 

 that there is no annual increase in 

 the diameter of the woody stems, 

 and no extensive branching, but 

 to this there are some exceptions. 



(3) Leaf veins forming a closed 

 system (Fig. 333, figure to left). 

 As a rule there is an evident set 



of veins which run approximately parallel, and intricately 



branching between them is a system of minute veinlets not 



readily seen. The vein system does not end freely in the 

 376 



Fig. 332. Section of stem of 

 corn, showing the scattered 

 bundles, indicated by black 

 dots in cross-section, and by 

 lines in longitudinal section. 

 —From " Plant Relations." 



