ITS INTERNAL STRUCTURE. 



113 



circles of wood, sun-ounding each other and a distinct pith, and all 

 surrounded by a separable bark. This is the plan, not only of the 

 Oak, but of all the trees and shrubs of the colder climates. In the 

 second, the woody system is not disposed in layers, but consists of 

 separate bundles or threads of woody fibre, &c., running through the 

 cellular system Avithout ajiparent order ; and presenting on the cross- 

 section a view of the divided ends of the.-e threads in the form of 

 dots, diffused through the whole ; but witli no distinct pith, and no 

 bark which is at any time readily separable from the wood. The 

 appearance of such a stem, both on the longitudinal and the cross- 

 section, is shown in Fig. 183 ; it may also be examined in the Cane 

 or Rattan, the Bamboo, and in the annual stallv of Indian Corn or 

 of Asparagus. That of ordinary wood of the first sort is too famil- 

 iar to need a pictorial illustration. 



200. Exogenous Structure. The stem, in the first case, increases 

 in diameter by the annual formation of a new layer of wood, which 

 is deposited between the preceding layer and the bark ; that is, the 

 wood increases by annual additions to its outside. Hence, such 

 stems are said to be Exogenous ; and plants whose stems grow in 

 this way are called Exogenous Plants, or briefly Exogens ; 

 that is, as the term literally signifies, outside-groivers. 



201. Endogenous Structure. In the second case, the new woody 

 matter is intermingled with the old, or deposited towards the centre, 

 which becomes more and more occupied 

 with the woody threads as the stem grows 

 older ; and increase in diameter, so far as 

 it depends on the formation of new wood, 

 generally takes place by the gradual dis- 

 tention of the whole. Accordingly, these 

 stems are said to exhibit the Endoge- 

 nous structure or growth ; and such plants 

 are called Endogenous Plants, or Endogens ; literally, inside- 

 growers. 



202. The two great classes of Phjenogamous plants, indicated by 

 this difference in the stem, are distinguishable even in the embryo 

 state, by differences quite as marked as those which prevail in their 

 whole port and aspect. The embryo of all plants that have en- 

 dogenous stems bears only a single cotyledon ; hence, Endogens are 

 also called Monocotyledonous Plants (128). Tlie embryo of 



FIG. 183. Section (longitudinal and transverse) of a Palm-stem. 



10* 



