346 PRIMARY DIVISIONS OF THE NATURAL SYSTEM. 



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 importance of the 

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

 may be considered as of more importance than the reproductive func- 

 tion, as being more essential for the life of the individual; and an 

 organ of first-rate value in the one, will therefore assume a higher func- 

 tion 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 re- 

 productive functions. This gives the true notion of affinity; and clas- 

 sifications formed 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. 



723. Primary 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 (*H 8); while others consist of 

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

 (^[ 28). 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 acotyledonous (^[ 590); the former being 

 divisible into monocotyledonous, having one cotyledon, and dicotyledo- 

 nous, having two cotyledons. The radicle, or young root of acotyle- 

 dons, is heterorhizal(^ 629), that of monocotyledons is endorhizal(^ 628), 

 that of dicotyledons, exorhizal (*([ 629). When the stems are taken 

 into consideration, it is seen that marked differences occur here also, 

 acotyledons being acrogenous, monocotyledons endogenous, and dicoty- 

 ledons exogenous (^[ 107). The venation of the leaves, whether parallel 

 or reticulated (^[ 143), establishes the same great natural divisions; and 

 similar results are obtained from a consideration of the flowers, mono- 

 cotyledons and dicotyledons being phanerogamous, and acotyledons 

 cryptogamous (^ 323). 



724. Thus, the following grand natural divisions are arrived at: 



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



2 Vascular /Monocotyledon ous. Endorhizal. Endogenous. > p hanero , amous 

 lar " \Dicotyledonous. ExorhizaL Exogenous. / 



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. 



725. Henslow illustrates the divisions and subdivisions of a natural 

 system by reference to Anihyllis Vulneraria, thus: 



