GROWTH. 131 



fibrous and vascular tissue running down the stem, and 

 is imbedded in the general cellular substance, each 

 bundle keeping distinct from its neighbour, and no con- 

 tinuity of them exists, so as to give rise to the compact 

 uniform layers seen in Exogens, however close the fibres 

 may happen to be, as in the Palms. 



209. No medullary rays similar to those of Exogens, 

 and no true bark enveloping the whole structure of the 

 stem is to be found. 



210. Close relationships have been endeavoured to 

 be established by some between the Endogenous and 

 Exogenous stem, medullary rays have been said to 

 exist in the Aloe ; and the rhizoma of a Tamus, ac- 

 cording to Du Trochet and Mohl grows like an Exo- 

 gen, whilst the ascending stem preserves the organi- 

 zation of an Endogen ; and, says Mohl, speaking of 

 the Rhizomse of Tamus elephantes, and T. communis, 

 " there is a central parenchymatous body abundantly 

 filled with amylaceous matter, in which run but few vas- 

 cular bundles, and a parenchymatous bark formed exte- 

 riorly of a thin layer of corky cellules." M. Decaisne 

 has shown, that a great number of Exogenous plants 

 are completely destitute of liber in certain parts, and 

 Phytolacca dioica in the whole of its stem, and as the 

 liber is always wanting in Endogens, so an analogy is 

 supposed to be established ; and finally, according to 

 Mohl, the stems of Endogens and Exogens are the 

 same during the first year of their growth. 



211. In respect to these and similar facts, which do 

 not admit of discussion in an elementary work, we may 

 use the words of Steinheil, " That without doubt a new 

 course has been opened to observation, and there are a 

 series of facts yet incomplete, which require to be con- 

 nected by some general formula ;" but at present we 

 must believe, that Endogenous and Exogenous stems 

 are distinct from each other. 



212. According to the famous hypothesis of Des 

 Fontaines, the increase of size of the Endogenous stem 

 takes place thus : At the centre of the stem, formed at 



