33.] 



CHAPTER II. THE TISSUES. 



177 



TTL 



rest of the primary phloem, but their cavities soon become 

 obliterated, so that they then look like strands of swollen cell- 

 wall (Fig. 137). 



The details of the differentiation of the primary vascular tissue 

 are essentially the same as in the case of the secondary vascular 

 lissue described on p. 202. 



The longitudinal differentiation of the primary vascular tissue does 

 not take place in the same order in all cases. In roots, and in 

 stems with cauline vascular tissue, the longitudinal differentiation 

 proceeds acropetally. In stems with common bundles the differ- 

 entiation usually begins in the procambium-strand at a node, pro- 

 ceeding both downwards 

 in the internode of the 

 stem, and outward into 

 the young leaf. 



In the majority of in- 

 stances, the whole of the 

 procambium - strand be- 

 comes differentiated into 

 permanent tissue, either 

 wood or bast ; this is true 

 for all roots, and for the 

 stems of nearly all Pterido- 

 phyta and Monocotyledons 

 (Fig. 137). Bundles of this 

 kind are said to be closed. 



In the Stems of most FlG- 130. Transverse section of the central part 



GymnOSpermS and DlCO- of tne root of Acorus Calamus (after Strasbnrger : 



, T -, ,1 ,, x 90) ; c lacunar cortex; eendodermis ; p pericycle; 



tyledons, On the Other spriinarywood . bund ies, with the small spiral vessels 



hand, the whole of the (protoxylem) externally; v bast bundles ; m pith; 



. the arrangement of the bundles is radial. 



procambium is not con- 

 verted into the primary wood and bast of the collateral conjoint 

 bundle, but a portion of it persists as an embryonic merismatic 

 tissue, the cambium, forming a transverse zone between the wood 

 on the inner (central) side and the bast on the outer side (see 

 Figs. 130, 139). Such a bundle is said to be open. 



Some few Dicotyledons have closed bundles (i.e. no cambium) in the stem, 

 e.g. Adoxa, Ranunculus Ficaria, Nymphseaceas, Myrlophyllum, Utricularia, etc. 



The position of the protoxylem and of the protophloem in the trans- 

 verse section of the bundle is not the same in the different members 



v. s. r, N 



