176 



PART II. THE INTIMATE STRUCTURE OF PLANTS. 



[33. 



merismatic cells, the procambium or primary desmogen (see p. 

 151) ; each procambium-strand of the plerome becomes a vascular 

 bundle of the stele. 



The development of the vascular tissue does not take place 

 simultaneously throughout the whole transverse section of the 

 procambium-strand, but begins 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 pro- 

 cambium cells 

 become spiral- 

 ly thickened 

 and lignified, 

 and the proto- 

 plasmic con- 

 tents of the 

 cells disap- 

 pear. It is 

 an important 

 generalisation 

 that spiral 

 or annular 

 vessels (or 

 tracheides) are 

 characteristic 

 of, and abso- 

 lutely confined 

 to, the proto- 

 xylem 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 tracheae presenting one or other of the various kinds 

 of pitted marking (p. 104). 



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

 conjoint bundle) begins with the differentiation of a small group 

 of sieve-tubes and companion-cells, constituting the protophloem, 

 which do not, however, differ in any marked manner from the 



FIG. 135. Part of a transverse section of the stele of the Sar- 

 saparilla-root (Smilax) : r cortex ; ed, endodermis with passage- 

 cells d; the pericycle and the interfascicular conjunctive tissue v 

 are sclerenchymatous ; v' the pith ; x the protoxylem, and t a pitted 

 vessel of a wood-bundle: s a bast-bundle. The alternation, or 

 radial arrangement, of the wood and bast-bundles is shown. ( x 

 300.) 



