234 SYSTEMATIC BOTANY 



By means of a Flora determine the species carefully and 

 try to arrange the plants found in the order of importance 

 in the vegetation : first the dominant ones, then the fre- 

 quent and occasional ones, and so on in descending order. 

 A collection of the characteristic species should be made 

 and classified according to habitat. Each should bear 

 a label giving the following particulars : natural order, 

 genus, species, plant-association or society, position in the 

 association (i. e. dominant or otherwise), locality, and date. 



Study in the same way the plants of several different 

 habitats, and contrast the types. In time you will learn 

 to recognize all the more important species and the more 

 interesting facts concerning their distribution. 



CHAPTER XIX 

 CLASS I, DICOTYLEDONS 



Archichlamydeae 



Dicotyledons are distinguished by having the vascular 

 bundles of the stem arranged in a ring, the leaves are 

 net-veined, the parts of the flower are in whorls of four 

 or five, and the embryo of the seed has two cotyledons. 



This class is much larger than that of the Monocotyle- 

 dons. The broad, net-veined leaf is very characteristic ; 

 secondary thickening is general. Dicotyledonous trees 

 form the great deciduous forests of Temperate regions ; 

 the evergreen shrubs and peculiar xerophytes of semi- 

 desert and desert regions belong largely to this class, 

 and many of the species forming the rank vegetation of 

 Tropical forests are also Dicotyledons. 



