126 PART II. ANATOMY AND HISTOLOGY. [ 33 



indication of the development of vascular tissue in the plerome 

 is the differentiation of one or more strands of narrow elongated 

 merismatic cells, the procambium (p. 116) ; each procambium- 

 strand_pf the plerome becomes a vascular bundle of thejitele. 



The development of the vascular tissue does not take place 

 simultaneously throughout the whole transverse section of the 

 procambium-strand, but bgins at one definite point, and extends 

 in one or more directions from that point. 



The development of the xylem-bundle (or part of a conjoint 

 bundle) begins with the differentiation of one or a few tracheids 

 or tracheae, constituting the protoxylem ; the walls of the corre- 

 sponding procambium-cells become spirally thickened and liguified, 

 and the protoplasmic contents of the cells disappear. It is an 

 important generalisation that spiral or annular vessels (or tra- 

 cheides) are characteristic of, ' and absolutely confined, to, the 

 protoxylem of the bundle. The remainder of the primary wood 

 (i.e. the wood which is developed from the procambium) is then 

 gradually differentiated, the walls of the tracheides or trachese 

 presenting one or other of the various kinds of pitted marking 

 (p. 74). 



Similarly, the development of the phloem-bundle (or the phloem 

 of a conjoint bundle) begins with the differentiation of a small 

 group of sieve-tissue, constituting the protophloem, which does 

 not, however, differ in any marked manner from the rest of the 

 primary phloem, but their cavities soon become obliterated, so 

 that they then look like strands of swollen cell-wall (Fig. 

 103). 



The details of the differentiation of the primary vascular tissue 

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

 tissue described on p. 145. 



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 differenti- 

 ation proceeds acropetally. In stems with common bundles the 

 differentiation usually begins in the procambium-strand at a node, 

 proceeding both downwards in the internode of the_stem, and out- 

 ward into the young leaf. 



In the majority of instances, the whoU of the procambium-strand 

 becomes 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. 103). Bundles of this kind are 



