lvi INTRODUCTION. 



important, toothed and entire leaves being often found in the same order. 

 Nevertheless, there is no instance of toothed leaves in Cinchonaceae, Gen- 

 tianeae, Guttiferae, or Malpighiaceae ; and they are very rare in Endoge- 

 nous 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 par- 

 ticular attention of botanists. It is already known that the internal 

 structure and peculiar growth of Exogenae and Endogense are externally 

 indicated by the arrangement of the veins of their leaves, — those of Exo- 

 genae diverging abruptly from the midrib, and then branching and anas- 

 tomosing in various ways, so as to form a reticulated plexus of veins of 

 unequal size ; while those of Endogenae 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 Endogenae ; all Myrtaceae have one or two fine 

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

 have the principal lateral veins running straight out from the midrib to the 

 margin ; Betulineae are distinguished by this among other characters from 

 Salicineae; and the same peculiarity separates the genuine genera of 

 Dilleniaceae, called Delimaceae by Decandolle, from those of which Hib- 

 bertia is the representative. — Leaves which contain reservoirs of oily secre- 

 tions, indicated by the presence of pellucid glands within their substance, 

 are almost always universal in a given order. Thus, Myrtaceae, properly 

 so called, (with the exception of the paradoxical pomegranate;) are distin- 

 guished by these glands from Melastomaceae ; in one genus of which, 

 however, (Diplogenea,) slight traces of them are to be found : they are 

 present in all Aurantiaceae ; by this character Wintereae are distinguished 

 from Magnoliaceae, Amyrideae from Connaraceae, &c. &c. In the orders 

 Phytolacceae, Petiveraceae, Labiatae, and Zygophylleae, there are, however, 

 genera with and without pellucid dots. 



At the base of some leaves are frequently found little membranous or 

 foliaceous appendages called Stipule, which are in fact leaves in an 

 imperfect state of developement. Their presence may therefore be under- 

 stood to indicate a peculiar degree of composition in the leaves to which 

 they belong, and they really indicate affinities in a very remarkable man- 

 ner. In studying them, however, care must be taken not to confound 

 genuine foliaceous appendages, to which alone the name of stipulae properly 

 appertain.-, with dilatations, or membranous or glandular processes of the 

 petiole, such as are found in Ranunculaceae, Grossulaceae, Apocyneae, 

 Umbellifcni', and others. The presence of stipulae is universal in Cincho- 

 naceae, which are thus distinguished from Stellatoe, in Betulineae, 



