lii INTRODUCTION. 



the pain of a poisoned limb, had he been acquainted with the laws of affi- 

 nity ; and, finally, the phenomena of grafting, that curious operation, which 

 is one of the grand features of distinction between the animal and vegeta- 

 ble kingdoms, and the success of which is wholly controlled by ties of 

 blood, can only be understood by the student of the Natural System. 



In every kind of arrangement, which has the natural relationship of 

 objects for its basis, there are are two principal inconveniences to overcome. 

 The first is, that as objects resemble each other more or less in a multitude 

 of different respects, it is impossible to indicate all their affinities in a lineal 

 arrangement ; and yet no other arrangement than a lineal one can be 

 practically employed. The consequence of this is, that while the orders 

 themselves are really natural, the same title often cannot be applied to the 

 arrangement of them in masses. For example, Cupuliferae and Betulineae 

 are obviously connected by the most intimate relationship, and, as collec- 

 tions of species, each of them is perfectly natural ; yet one of them stands 

 among Apetalous plants, the other among Achlamydeous ones ; hence 

 the two latter groups are artificial. In fact it appears from what we at 

 present know, that no large combinations of orders are natural which are 

 not founded upon anatomical differences ; thus, Cellulares and Vasculares, 

 Exogenae and Endogenae, Gymnospermous and Angiospermons Dicoty- 

 ledons, are natural divisions ; but Apetalae, Polypetalae, Monopetalae, 

 Achlamydeae, and all their subordinate sections, are entirely artificial. 



The second inconvenience is, that the characters which vegetables 

 exhibit are of such uncertain and variable degrees of importance, that it is 

 often difficult to say what value should be attached to any given modifi- 

 cation of structure. As this is a practical question, which requires to be 

 well understood, I shall endeavour to explain in some detail the nature 

 and relative value of those peculiarities of which botanists make use in 

 determining vegetable affinities ; repeating, as a general rule which is 

 not open to exception, that characters which are purely physiological, — 

 that is to say, which depend upon differences of internal anatomical struc- 

 ture, — are of much more value than varieties of form, position, number, 

 and the like, which are mere modifications of external organs. 



It is a maxim of the Linnsean school, that the parts of fructification should 

 be employed in characterizing classes, orders, and genera, to the exclusion 

 of all modifications of the leaves or stem. This, although theoretically 

 insisted upon, was practically abandoned by Linnaeus himself, and is to 

 be received with great caution. The organs of fructification are only enti- 

 tled to a superior degree of consideration, when found by experience to be 

 less liable to variation than those of vegetation. 



All plants are composed of what are called elementary organs, that is to 

 say, of a vegetable membrane appearing under the form of parenchyma 

 or cellular tissue in different states, of spiral vessels, and of ducts, or tubes : 

 these organs enter into the composition of plants in various ways, and are 

 not all even necessary to their existence : sometimes spiral vessels disap- 

 pear, and again both these and the ducts cease to be developed, — cellular 

 tissue, which is the basis of vegetation, alone remaining. Upon the pecu- 

 liar arrangement of these minute organs, external form necessarily 

 depends ; and as it is found by experience, that while the anatomical 

 structure of plants is subject to little or no variation, it is difficult to define 

 their external modifications with accuracy, the reason of the superior im- 

 portance of physiological characters will be apparent. 



