CLA SSIFICA TION. 235 



a single character is not sufficient to show relationship in the 

 class (nor is it in orders, nor in many of the lower grades), but 

 one must use the sum of several important characters. 



383. Dicotyledones. — In the dicotyledons there are two 

 cotyledons on the embryo ; the venation of the leaves is reticu- 

 late; the endosperm is usually absent in the seed; the parts of the 

 flower are frequently in fives; the vascular bundles of the stem 

 are generally open and arranged in rings around the stem as shown 

 in the cross-section of the oak (fig. 204). There are exceptions 

 to all the above characters and the sum of the characters must 

 be considered, just as in the case of the monocotyledons. 



384. Taxonomy. — This grouping of plants into species, 

 genera, families, etc., according to characters and relationships 

 is classification, or taxonomy. 



To take Trillium grandiflorum for example, its position in 

 the system, if all the principal subdivisions should be included 

 in the outline, would be indicated as follows: 

 Group, Angiosperms. 

 Class, Monocotyledones. 

 Order, Liliiflora. 

 Family, Liliaceae. 

 Genus, Trillium. 



Species, grandiflorum. 

 In the same way the position of toothwort would be indicated 

 as follows : 



Group, Angiosperms. 

 Class, Dicotyledones. 

 Order, Rhceadinse. 

 Family, Cruciferse. 

 Genus, Dentaria. 

 Species, diphylla. 

 But in giving the technical name of the plant only two of 

 these names are used, the genus and species, so that for the 

 toothwort we say Dentaria diphylla, and for the white wake- 

 robin, we say Trillium grandiflorum. 



