414 



STRUCTURE OF DICOTYLEDONOUS STEM. 



Fig. 219. 



very correctly to the mode of increase of the Woody layers, although 

 (as just shown) the Liber is formed upon a truly Endogenous plan. 

 304. Numerous departures from the normal type are found in 

 particular tribes of Exogens. Thus in some the Wood is not 

 marked by concentric circles, their growth not being interrupted 

 by any seasonal change. In other cases, again, each Woody zone 

 is separated from the next by the interposition of a thick layer of 

 Cellular substance. Sometimes Wood is formed in the Bark (as in 

 Calycanthus), so that several woody columns are produced, which 



are quite independent of 

 the principal woody axis, 

 but cluster around it. 

 Occasionally the Woody 

 Stem is divided into dis- 

 tinct segments by the 

 peculiar thickness of cer- 

 tain of the Medullary 

 Rays ; and in the Stem of 

 which Fig. 219 represents 

 a transverse section, these 

 Cellular plates form four 

 large segments, disposed 

 in the manner of a Mal- 

 tese cross, and alternating 

 with the four Woody seg- 

 ments, which they equal 

 in size. 



305. The Exogenous 

 Stem, like the so-called 

 Endogenous, consists in 

 its first-developed state of 

 Cellular tissue only; but 

 after the Leaves have 

 been actively performing their functions for a short time, we 

 find a circle of Fibro- Vascular bundles, as represented in the 

 Diagram, p. 408, interposed between the central (or Medullary) 

 and the peripheral (or Cortical) portions of the Cellular matrix ; 

 these Fibro-Vascular bundles being themselves separated from each 

 other by plates of Cellular tissue, which still remain to connect the 

 central and the peripheral portions of the matrix. This first stage 

 in the formation of the Exogenous axis, in which its principal 

 parts — the Pith, Wood, Bark, and Medullary Rays — are marked 

 out, is seen even in the stems of Herbaceous Plants, which are 

 destined to die down at the end of the season (Fig. 220) ; and 

 sections of these, which are very easily prepared, are most inte- 

 resting Microscopic objects. In such stems, the difference between 

 the Endogenous and the Exogenous types is manifested in little else 

 than the disposition of the Fibro-Vascular layers ; which are scat- 



Transverse section of the stem of a 

 climbing-plant {Aristolochia ?) from New 

 Zealand. 



