XXll INTRODUCTION,? 



I do not know how far this irregularity is connected with the follow- 

 ing observations of Schlechtendahl, which, however, deserve atten- 

 tion. " Those leaves," he says, " which are connected either by 

 their base, or by the intervention of a stipula, I call opposite, and 

 those which are not so connected, spuriously opposite (pseudo- 

 opposita). Opposite leaves are never disjoined, as in Rubiacea; 

 and Caryophylleae ; spuriously opposite ones, which are much more 

 common, being easily disjoined, readily become alternate. Branches 

 obey the same laws as leaves." Limiaa, 1. 207. — All Spondiacese, 

 Rhizoboleae, &,c. have compound leaves ; in many others they are 

 always simple ; and in such orders as Acerinese, Aurantiacea?, 

 Geraniacese, Rutaceae, and Sapindaceae, both simple and compound 

 leaves are found. This character, therefore, is not considered of so 

 much value as many others. — Neither is the degree of division of 

 the margin usually important, toothed and entire leaves being 

 often found in the same order. Nevertheless, there is no instance 

 of toothed leaves in Cinchonaceas, Gentianese, Guttifei'se, or Mal- 

 pighiaceae ; and they are very rare in Endogenous plants. — 

 Characters derived from the arrangement of veins are known to 

 be in many cases of the utmost importance ; and it is probable, 

 that when this subject shall have been more accurately studied, 

 they will be found of even more value than has been yet supposed. 

 The great obstacle to employing characters derived from venation, 

 exists in the want of words to express clearly and accurately the 

 different modes in which veins are arranged. I have endeavoured 

 to remove this by some observations in the Botanical Register; 

 and I am persuaded the subject deserves the particular attention 

 of botanists. It is already known that the internal structure and 

 peculiar growth of Exogense and Endogenae are externally indi- 

 cated by the arrangement of the veins of their leaves, — those of 

 Exogenae diverging abruptly from the midrib, and then branching 

 and anastomosing in various ways, so as to form a reticulated plexus 

 of veins of unequal size ; while t^ose of Endogenoi run straight from 

 the base to the apex, or diverge gradually from the midrib, not 

 ramifying in their course, but being simply connected with each 

 other by transverse bars, examples of which are afforded on the one 

 hand by the Rose, and on the other by the Iris and Arrow-root. 

 Although a few exceptions exist to both these laws, yet the grand 

 characters of the leaves of those classes are such as I describe. But, 

 independently of this, many other orders are distinguished without 

 exception by modifications of venation. Thus, all Melastomaceae 

 have three or more collateral ribs connected by branched transverse 

 bars, something in the way of Endogenai ; all Myrtaceiu have one or 

 two fine veins running parallel with the margin, and just within it ; all 



