liv INTRODUCTION. 



Therefore, a section of the trunk of an Exogenous plant exhibits bark on 

 the outside, pith in the centre, and concentric deposits of woody matter 

 between these two, all connected in a solid mass by plates of cellular tissue, 

 radiating from the centre to the circumference, and called medullary rays. 

 Endogenae, the addition to which is internal, have no need of an external 

 coating to protect their newly formed matter from injury, and are there- 

 fore destitute of bark ; moreover, as the layers of new matter are not con- 

 centric, but irregular, and do not either correspond with particular seasons 

 of growth, nor commence round any distinct centre of vegetation, there is 

 no distinction of bark, woody concentric deposits, and pith ; the connecting 

 tissue by which the parts are all tied together is mixed up with the sub- 

 stance 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 Exogenae I do not know that any remarks need be made, they 

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

 in Coniferse and Cycadeae, which, without deviating from the mode of 

 growth of Exogenae, exhibit a peculiar modification of the woody tissue. 

 But Endogenee are perhaps divisible into two subordinate forms, which 

 have been pointed out by Agardh. First, Grasses, which, as this distin- 

 guished writer well remarks, are the least monocotyledonous of all ; they 

 have a distinct pith, hollow branched stems like Umbelliferee, and buds at 

 the axillae 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 distin- 

 guished into such as are Exogenous and such as are Endogenous ; 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. Vegeta- 

 ble anatomy has not yet been studied sufficiently with a view to generali- 

 zation, 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 differ- 

 ences far beyond what has yet been observed. 



Anatomical differences in plants having been apparently exhausted, 

 inquiry has been turned to the degree in which modifications of the com- 

 pound 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 fre- 

 quently 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 Endo- 

 genae they are sometimes characteristic of particular orders. Thus, all 

 Marantaceee and Scitamineee, and most Iridese, have a rhizoma in one 



