PEIMAKY DIVISIONS OF THE NATURAL SYSTEM. 417 



way the other values are determined. In examining organs it is 

 essential to compare those which belong to the same series ; for an 

 organ which occupies the highest place in one series may be inferior 

 in value to a second-rate organ in another. The comparative import- 

 ance of the different series must be taken into account also. Thus, 

 the nutritive may be considered as of more importance than the re- 

 productive function, as being more essential for the life of the in- 

 dividual ; and an organ of first-r^te value in the one wiU therefore 

 assume a higher function than one of the same value in the other. 

 The changes which take place in any one set of organs are often 

 accompanied with changes in others ; and thus it is found that natural 

 divisions may be arrived at by different routes— for instance, by the 

 elementary, nutritive, and reproductive functions. This gives the 

 true notion of afSnity ; and classifications founded on such principles 

 will obviously be more valuable, in a practical and physiological point 

 of view, than those which adopt characters in an arbitrary manner. 



Peimaby Divisions of the Vegetable Kingdom. — In taking 

 a survey of the Vegetable Kingdom, some plants are found to be com- 

 posed of cells only, and are called Cellular (p. 5) ; while others consist 

 of cells and vessels, especially spiral vessels, and are denominated 

 Vascular (p. 16). If the embryo is examined, it is found in some 

 cases to have cotyledons or seed-lobes, in other cases to want them ; 

 and thus some plants are cotyledonous, others acotykdonous (p. ■334) ; 

 the former being divisible into mmocotyledonous, having one cotyle- 

 don ; and dicotyledonous, having two cotyledons. The radicle, or young 

 root of acotyledons, is heterorhiml (p. 357), that of monocotyledons is 

 endorhizal (p. 356), that of dicotyledons exorhizal (p. 357). When 

 the stems are taken into consideration, it is seen that marked differ- 

 ences occur here also, — acotyledons being acrogenous, monocotyledons 

 endogenous, and dicotyledons exogenous (p. 75). The venation of 

 leaves, parallel, reticulated, or forked (p. 84), establishes the same 

 great natural divisions ; and similar results are obtained from a con- 

 sideration of the flowers, — monocotyledons and dicotyledons being 

 phanerogamous, and acotyledons cryptogamous (p. 171). 



Thus, the following natural divisions are arrived at : — 



1. Cellular . Acotyledonous. Heterorhizal. Acrogenous. Cryptogamous. 



„ „ , I Monocotyledonous. Endorhizal. Endogenous. ) ■□, 



2. Vascular . | oieotyledonous. Exorlmal. Exogenous. [ Pl^anerogamous. 



These larger groups are, on similar principles, subdivided, until at 

 length genera and species are reached by a process of analysis. 

 Similar results will be obtained by a synthetical process, conducted 

 on the same principles, and proceeding from species upwards. 



Henslow illustrates the divisions and subdivisions of a natural 

 system by reference to AnthylUs Vulneraria, thus : — 



2e 



