INTRODUCTION. XIX 



Vasculares, or those which have spiral vessels, being the same as 

 Sexual plants, and Cellulares, or those which have no spiral 

 vessels, answering to Asexual plants. 



Sexual organs being considered essential to a flower (no apparatus 

 whatever from which they are absent being understood to constitute 

 one), two other unexceptionable characters belong to these same divi- 

 sions ; all Vasculares, or Sexual plants, bear flowers, and all Cellulares, 

 or Asexual plants, are flowerless ; the former are also called Phaeno- 

 gamous, the latter Cryptogamous. 



Two great but unequal divisions being thus established, upon 

 both anatomical and external characters, botanists have inquired 

 whether similar differences of a secondary character could not be 

 discovered among each of them. Observations upon Cellulares have 

 led to the establishment of three groups of unequal importance, 

 which are not, however, universally received. Vasculares have been 

 found to comprehend two great but unequal tribes, differing essen- 

 tially in the laws which govern their growth. It has been ascertained 

 that a large number of them grows by the addition of successive 

 layers of new matter to the outside, and that another, but smaller 

 number, increases by additions to the inside ; the youngest or most 

 newly formed parts being in the one case on the outside, and in the 

 other case in the inside. For this reason, one of these divisions has 

 been called Exogenous, and the other Endogenous. It is difficult 

 to conceive how the external increase of Exogenffi could take place 

 without some adequate protection to the young newly formed tissue 

 from the atmosphere and accidental injury, and, accordingly, the 

 substance called bark is created by nature for that purpose, within 

 which the new deposit takes place : as this last is formed annually, 

 the age of an Exogenous plant is indicated in the trunk by imaginary 

 lines called concentric circles, which are in fact caused by the cessa- 

 tion of growth in one year, and the renewal of it in another. The 

 centre of this system is a cellular substance called pith. 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. Endogense, the addition to which is internal, have 

 no need of an external coating to protect their newly formed matter 

 from injury, and are therefore destitute of bark; moreover, as the 

 layers of new matter are not concentric, 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 substance 



