114 THE STEM. 



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

 iar to need a pictorial illustration. 



186. 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 ; in other 

 words, the wood increases by annual additions to its outside. 

 Hence, such stems are said to have the EXOGENOUS structure ; and 

 the plants whose stems grow in this way are called EXOGENOUS 

 PLANTS, or briefly EXOGENS ; that is, as the term literally signi- 

 fies, outside- growers. 



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

 matter is intermingled with the old, or deposited towards the cen- 

 tre, 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 distention of the whole, the new wood pushing the 

 old outwards. Accordingly, these stems are said to exhibit the 

 ENDOGENOUS structure or growth ; and such plants are called EN- 

 DOGENOUS PLANTS, or ENDOGENS ; literally, inside- growers. 



188. The two great classes of Phsenogamous plants, indicated 

 by this difference in the stem, are distinguishable even in the em- 

 bryo state, by differences quite as marked as those which prevail 

 in their whole port and aspect. The embryo of all plants that 

 have endogenous stems bears only a single cotyledon, and there- 

 fore sends up but one seed-leaf in germination ; hence, Endogens 

 are also called MONOCOTYLEDONOUS PLANTS. The embryo of 

 plants with exogenous stems bears a pair of cotyledons and un- 

 folds a pair of seed-leaves in germination (Fig. 105- 107) : hence 

 Exogens are likewise called DICOTYLEDONOUS PLANTS. 



SECT. V. THE EXOGENOUS OR DICOTYLEDONOUS STEM. 



189. Since the Exogenous class is by far the largest in every 

 part of the world, and embraces all the trees and shrubs with 

 which we are familiar in the cooler climates, the structure of 

 this kind of stem demands the earlier and more detailed notice. 

 To obtain a true and clear idea of its internal structure, we should 

 commence at its origin and follow the course of development. 



190. In the embryo state, or at least at some period antecedent 

 to germination, the rudimentary stem is entirely composed of pa- 



