NOMENCLATURE. 367 



695. These five classes are very unequal, in respect to the 

 number of plants they embrace ; the Exogenous class containing 

 much the largest number of species as well as orders ; the Endo- 

 gens also comprising numerous types ; but the others very few in 

 comparison. Convenience of analysis therefore requires that the 

 larger classes should be broken up into divisions, alliances, co- 

 horts, or by whatever name groups intermediate between the 

 classes and orders may be termed : and the accomplishment of 

 this object, so as to form natural groups, is at present the great de- 

 sideratum in Systematic Botany. But until this be well done, we 

 are obliged to use artificial analyses of the classes, or to throv the 

 orders into groups, which, in proportion as they are rendered natu- 

 ral, it becomes impossible strictly to circumscribe. In this view, 

 the great class of Exogenous plants is usually broken up into three 

 very convenient, but nearly artificial portions, founded on the pres- 

 ence, absence, or union of the petals ; namely : 



1. POLYPETAL^E, the Polypetalous Exogens ; where the calyx 



and corolla are both present, and the latter composed of 

 distinct petals. 



2. MONOPETAL^ or" GAMOPETALJS, the Monopetalous Exogens ; 



where the petals are united. 



3. APETALJE, the Apetalous Exogens ; where the petals are 



wanting, and the floral envelopes, if present at all, consist 

 of the calyx alone. 



696. These divisions, as well as the other classes, are subdi- 

 vided by different authors in various ways, which need not be spe- 

 cified ; since it is only the classes and the orders that are consid- 

 ered to rest upon a stable basis. 



697. The orders, or families, are to be viewed rather as natural 

 groups of genera, than as subdivisions of the classes. The kind of 

 characters employed in distinguishing them will best be learned 

 from the succeeding illustrations. 



698. Nomenclature, Their names, which are always plural, some- 

 times express a characteristic feature of the group ; as, for in- 

 stance, Leguminoscc, or the Leguminous plants, such as the Pea, 

 Bean, &c., whose fruit is a legume (603) ; UmbeUifera, or Um- 

 belliferous plants, so named from having the flowers in umbels ; 

 Composite, an order having what were termed compound flowers 

 by the earlier botanists (394) ; Labiata, so called from the labiate 



