II. ORDERS AND GENERA. 19 



and species; genera being smaller groups than orders, 

 and composed of more closely allied species. This ex- 

 planation may be regarded as true in the main. But it 

 is also true, that genera are groups formed on a different 

 principle from orders and species. They are dissimilar 

 in kind as well as in degree. In combining individuals 

 or varieties into species, the whole plant and all its parts 

 are considered ; and more than this, differences of dura- 

 tion and texture, as annual or perennial, herbaceous or 

 ligneous, are also taken into account. But in grouping 

 species into genera, another rule is usually well adhered 

 to ; that of looking for generic characters solely in the 

 structure of the flowers and fruit. Genera are thus con- 

 stituted on more partial and eclectic characters than are 

 species. But when these genera, — or, rather, their 

 included species, — are to be grouped into orders, a 

 widened choice of characters is resorted to ; not so wide 

 as in the case of species, but much wider than in the case 

 of genera. For example, the presence or absence of sti- 

 pules, the alternate or verticillate disj)osition of the 

 leaves, their venation and other peculiarities, the angu- 

 lar or terete form of the stem, &c. &c., are occasionally 

 resorted to as ordinal characters, though not used as 

 generic characters. 



It is thus clear that orders, genera, and species are 

 not strictly major and minor groups of plants, formed on 

 uniform principles, but groups constituted on varying 

 principles. The uniformity would be iBore close if the 

 characters of orders were taken from the reproductive 

 organs only, — or if genera had been formed and defined 

 on more extended characters, truly intermediate between 

 those looked to for orders and species respectively. And 

 for the purposes of geographic botany, it is inconvenient 

 that genera are not thus instituted on characters drawn 



