XX INTRODUCTION. 



of the whole, and does not radiate regularly in plates from the centre 

 to the circumference, and consequently there are no medullary rays. 

 Nothing can be more clearly made out than the existence of these 

 two modes of growth in vascular plants ; and the nature of them 

 will be at once understood by an inspection of a section of an Oak 

 branch, and of a Cane. 



Upon Exogense I do not know that any remarks need be made, 

 they being exceedingly uniform in the great features of their structure ; 

 except in Coniferae and Cycadeae, which, without deviating from the 

 mode of growth of Exogenae, exhibit a peculiar modification of the 

 woody tissue. But Endogenae are perhaps divisible into two sub- 

 ordinate forms, which have been pointed out by Agardh. First, 

 Grasses, which, as this distinguished writer well remarks, are the 

 least monocotyledonous of all ; they have a distinct pith, hollow 

 branched stems like Umbelliferae, and buds at the axilla of the leaves ; 

 but they have no bark, no medullary rays, and their direction of 

 increase is inwards : and, secondly. Palms, which are endogenous in 

 the strictest sense of the word. 



From this it appears, that Vasculares, or Flowering plants, are 

 distinguished into such as are Exogenous and such as are Endo- 

 genous ; and that while the former are incapable of any further 

 anatomical division, the latter contain perhaps two different forms. 

 It must, however, be borne in mind, that a great deal is yet to be 

 learned upon this subject Vegetable anatomy has not yet been 

 studied sufficiently with a view to generalization, and is, besides, a 

 subject yet in its infancy. Nothing can be more probable than that 

 differences in the tissue, or in the relative position or structure of 

 vessels, will one day be found to accompany external differences far 

 beyond what has yet been observed. 



Anatomical differences in plants having been apparently ex- 

 hausted, inquiry has been turned to the degree in which modifi- 

 cations of the compound or external organs are capable of being 

 employed to determine natural affinities ; and it has been found that 

 these, although of secondary importance only, nevertheless deserve 

 the utmost attention-, as they frequently afford the only characters of 

 which it is practicable to make use. 



The Root, properly so called, offers no characters that have 

 been found uniform in particular families ; in fact, the modifications 

 of which it is susceptible are so few, that it is difficult to conceive 

 in what way they can be applied. Certain forms of root-like stems 

 and buds have, however, been observed ^ to which some attention 

 should be paid. In the first place, neither bulb nor rhizoma is known 

 in Exogenous plants, while in Endogenie they arc sometimes charac- 

 teristic of particular orders. Thus, all Marantaceic and Scitaminete, 



