230 NATURAL 



example, increase, by forming a new layer of woody matter 

 external to the old one, and between it and the bark, every 

 year. Hence the ages of Exogenous trees are indicated 

 by the concentric circles of which they are composed, and 

 which appear to be occasioned by the cessation of growth 

 during one period of the year, and the renewal of it in an- 

 other. The centre of this vegetable system, is a spongy, 

 or cellular substance, called the pith. Therefore, when 

 the trunk of the Oak, Ash, or any other perennial Exogen- 

 ous plant is sawn across, it exhibits bark on the outside, 

 pith in the centre, and concentric deposits of woody matter 

 between them, all connected by plates of cellular tissue, 

 radiating from the centre to the circumference, and called 

 medullary rays. These rays are very obvious on splitting a 

 piece of oak. 



Endogenous plants, of which the Lily, Palm, and Iris are 

 examples, have no need of bark, or other external covering, 

 to protect their newly formed parts from injury, since theii 

 additions are internal. In these plants, as the layers of new 

 matter are not concentric but irregular, and neither corres- 

 pond with particular seasons of growth, nor commence round 

 a pith, or any other distinct centre of vegetation, there are 

 no lines which distinguish the annual deposits from each 

 other, or these from the bark and centre, as in the Exogenous 

 species. In the Endogenae, these parts are all confounded : 

 in Exogenae, they are all distinct. 



The Exogenae, and Endogenae, are further distinguished 

 by a difference in their seeds, the first being all Dicotyledo- 

 nous, that is, consisting of two cotyledons, or seed lobes, 

 while the Endogenous tribe are all Monocotyledonous, or 

 have only a single seed lobe. In the Exogeriae, also, the 

 leaves are reticulated, or are formed like net work, while in 

 the Endogenae, the veins of the leaves run straight, and are 

 parallel. The Exogenae, or Dicotyledonous plants are sub- 

 divided into two tribes called Angiosperni(K,'and Gymnos- 

 permcB ; the first, denoting that the seeds are enclosed in a 

 pericarp, and the second, that the seeds are naked, or desti- 

 tute of a pericarp. 



The Endogenae, or Monocotyledonous plants, are also 

 separated into two tribes, the first of which is called Peta- 

 loidae, and includes such plants as have a calyx and corolla, 

 in three, or six divisions, or if these parts are absent, then 

 the stamens and pistils are naked The second tribe is 



