ORIGIN OF THE WOOD. 131 



towards the centre and most compact at the circumference. In 

 this way, and by the general growth of the cellular tissue in which 

 the fibro- vascular bundles are imbedded, the stem increases in di- 

 ameter as long as the rind is capable of distention. In some in- 

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

 trees, the rind remains soft and capable of unlimited expansion ; 

 and the woody bundles descend after having reached the circum- 

 ference, and thus the older stems continue to increase in diameter, 

 much after the manner of an Exogen ; but in the Palms, and in 

 most woody Endogens, it soon indurates, and the stem consequent- 

 ly increases no further in diameter. The wood of the lower part 

 of such stems is more compact than the upper, being more filled 

 with woody bundles, the cells of which are lignified by internal 

 deposition ; and the rind is harder, from the greater number of lig- 

 neous fibres which terminate in it, and from its proper induration. 

 Further increase in diameter being in these cases impossible, and 

 the lower part of the stem becoming at length choked up by the 

 multitude of descending bundles, it appears that the life of such 

 Endogens must be limited. 



222. Palms generally grow from the terminal bud alone, and 

 perish if this bud be destroyed ; the foliage is also borne in a clus- 

 ter at the summit of the trunk ; which consequently forms a simple 

 cylindrical column. But in some instances two or more buds de- 

 velope, and the stem branches, as in the Doum-Palm of Upper 

 Egypt, and in the Pandanus, or Screw-Pine (Fig. 117), which 

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

 branches are 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. 



223. Grasses have endogenous stems, mostly of annual dura- 

 tion, and which early become hollow in the manner already indi- 

 cated (193). In several of them, such as the Maize and Sugar- 

 Cane, the stem remains solid ; and these furnish good examples of 

 ordinary endogenous structure. 



SECT. VII. OF THE THEORETICAL STRUCTURE OF THE STEM, ETC. 



224. Origin Of the Wood, &C, We have seen that the plantlet 

 which has as yet developed only one internode and one leaf (188), 



