116 THE STEJr. 



produced), as long as the rind is capable of distention. In some 

 instances, as in the arborescent Yuccas and the Dracaenas or Dragon- 

 trees, the rind remains soft and capable of unlimited growth ; but in 

 the Palms, and in most woody Endogens, it soon indurates, and the 

 stem consequently increases no further in diameter. The wood of 

 the lower part of such stem is more compact than the upper, being 

 more filled with woody bundles, and the rind is firmer, from the 

 greater number of ligneous fibres which terminate in it, and from 

 its proper induration. 



206. Pahns generally grow from the terminal bud alone, and 

 perish if this bud be destroyed ; they grow slowly, and bear their 

 foliage in a cluster at the summit of the trunk, which consequently 

 forms a simple cylindrical column. But in some instances two or 

 more buds develop, and the stem branches, as in the Doura Palm of 

 Upper Egypt, and in the Pandanus, or Screw-Pine (Fig. 140), 

 which belongs to a family allied to Palms : in such cases the 

 branches ai'e cylindrical. But when lateral buds are freely devel- 

 oped (as in the Asparagus), or the leaves are scattered along the 

 stem or branches (as in the Bamboo, Maize, &c.), these taper up- 

 wards, just as in Exogens. A particular comparison of the structure 

 and growth of the Endogenous stem with the Exogenous cannot be 

 instituted until the latter is explained in detail. 



Sect. YI. The Exogenous or Dicotyledonous Stem. 



207. Since the Exogenous class is by far the larger 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 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. 



208. In the embryo, or at least at some period antecedent to 

 germination, the rudimentary stem is entirely composed of paren- 

 chyma. But as soon as it begins to grow, some of the cells begin to 

 lengthen into tubes, to be marked with transverse bars or spiral 

 lines, and thus give rise to ducts or vessels (57 - 60) ; tliese form a 

 small and definite number of bundles or threads, say four equidistant 

 ones in the first instance, as in the Sugar Maple : surrounding these, 

 other slender cells of smaller calibre, and destitute of markings, 



