STRUCTURE OF STEMS 709 



always to keep apart the liber and the alburnum. This type of 

 stem-structure is termed exogenous ; a designation which applies 

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

 (as just shown) the liber is formed upon a truly endogenous plan. 



.Numerous departures from the normal type are found in particu- 

 lar tribes of dicotyledons. 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 \vood is formed in the bark (as in Calycanthus), so that 

 several woody columns are produced, which are quite independent of 

 the principal woody axis, and cluster around it. Occasionally the 

 woody stem is divided into distinct segments by the peculiar thick- 

 ness of certain of the medullary rays, and in the stem, of which 

 fig. 554 represents a transverse section, these cellular plates form 



FIG. 554. Transverse section of the FIG. 555. Portion of transverse 



stem of a climbing plant (Aristo- section of Arctium (burdock), 



locJiia ?) from New Zealand. showing one of the fibro- vascu- 



lar bundles that lie beneath 

 the cellular epiderra. 



four large segments disposed in the manner of a Maltese cross, and 

 alternating with the four woody segments, which they equal in size. 

 The exogenous stem, like the (so-called) endogenous, consists, in 

 its first -developed state, of cellular tissue only ; but after the leaves 

 have been actiyely performing their function for a short time, we 

 find a circle of fibro-vascular bundles, as represented in fig. 540, 

 interposed between the central (or medullary) and the peripheral 

 (or cortical) portions of the fundamental tissue, 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 that tissue. 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. 555) ; and sections of these, which are very 



