XVlll IXTRODUCTION. 



cation should be employed in characterising 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 entitled to a superior degree of con- 

 sideration, 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 exist- 

 ence : sometimes spiral vessels disappear, and again both these and 

 the ducts cease to be developed, — cellular tissue, which is the basis of 

 vegetation, alone remaining. Upon the peculiar 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 diflicult to define their external modifi- 

 cations with accuracy, the reason of the superior importance of 

 physiological characters will be apparent. 



Some, and by far the greater part of, plants are propagated by 

 productions called seeds, which are the result of an action believed 

 to be analogous to the sexual intercourse of animals ; others are 

 multiplied by bodies called sporules, of the I'eal nature of which 

 little is yet known, further than that they do not appear to result 

 from the communication of sexes. Hence plants are naturally and 

 primarily divided into two great divisions, called Sexual and 

 Asexual. 



Physiologists have discovered that these peculiarities are con- 

 nected with others in anatomical structure of no less importance. 

 For instance, plants propagated by seeds, and possessing distinct 

 sexes, have spiral vessels ; while those which are increased by bodies 

 not depending upon the presence of sexual apparatus, are universally 

 destitute of spiral vessels. To the latter statement there is no known 

 exception, — species to which spiral vessels have been ascribed being 

 found to possess nothing more nearly related to those organs than 

 ducts, or false tracheae. The former character is not absolutely 

 without exception ; the singular genus llafflcsia being described both 

 by Brown and Blume as without spiral vessels, Caulinia fragilis not 

 having them according to Amici, and Lemna being destitute of them 

 according to the evidence of others. But these exceptions are not 

 regarded of much importance. 



It therefore appears that two great divisions, established upon 

 different principles, agree in the kind of plants they comprehend ; 



