398 THE SPERMATOPHYTES 



There are more than 20,000 species of monocotyledons, which 

 are arranged by Engler into 1 1 orders, the chief of which are : 



1. The grass and sedge order, Graminales, including more 

 than 6000 species, one of the most successful assemblages of 

 angiosperms and by far the largest in the number of individuals. 



2. The palm order, Palmales, a very characteristic tropical 

 and sub-tropical group. 



3. The lily order, Liliales, a large group of almost 5000 

 species, remarkable for the showiness and symmetry of its 

 flowers. 



4. The orchid order, Orchidales, containing the large orchid 

 family with more than 5000 species, the largest family in the 

 Monocotyledonece, and one of the most remarkable groups of seed 

 plants for the beauty and complexity of its flowers and for its 

 peculiar life habits. 



371. The dicotyledons. Besides having two cotyledons in the 

 embryo, this group is distinguished from the monocotyledons 

 by having its fibre-vascular bundles formed in a circle. This 

 arrangement makes possible the development of a central shaft 

 of wood (xylem), since the cambium regions of the bundles 

 unite into a cylinder which adds successive layers of wood if 

 the plant is perennial. The bundles in the leaves are strongly 

 developed, much branched, and end freely, .so that the leaves 

 are conspicuously open (netted) veined, generally notched, and 

 frequently deeply divided, or compound. The parts of the flowers 

 are mostly in fours and fives in the higher types, except that the 

 number of carpels is commonly less. 



There are more than 100,000 species of dicotyledons, and 

 these are arranged by Engler into 34 orders (26 in the Archi- 

 chlamydece, and 8 in the Metacldamydecc). 



372. The Archichlamydeae. This sub-class is an immense 

 assemblage, very diverse in character, whose flowers range from 

 primitive types, with indefinite numbers of parts in spiral arrange- 

 ments, to cyclic flowers with definite numbers, perigyny, epig- 

 yny, and syncarpy. Some of the chief orders are : 



