33] 



CHAPTER II. THE TISSUES. 



129 



dleSj each of the bundles of the root bifurcating above, so that 

 there are commonly twice as many bundles in the stem as in the 

 corresponding root. 



The structure of the primary bundle. The primary wood 

 (whether in an isolated or a conjoint bundle) consists essentially of 

 lignified tracheal tissue (tracheae, or tracheids, p. 93), together with 

 a varying proportion of wood-parenchyma, more or less lignified, the 

 cells being occasionally somewhat fibrous. The protoxylem (see p. 



cp 



n' 



FIG. 104. F?adial longitudinal section of a conjoint, closed, collateral bnndle from the 

 stem of a Monocotyledon (Zea Mais ; after Strasburger, x 180) ; to the right is the central 

 (medullary) limit of the bundle ; to the left the peripheral (cortical) limit : c p protophloem ; 

 v sieve-tubes of the bast, with companion-cells * ; p a a', the protoxylem ; a a' remains of 

 ruptured annular vessel lying in the lytigenous lacuna I ; vg sheaths of sclerenchymatous 

 conjunctive tissue. (Compare this with Fig. 103.) 



126) is usually a conspicuous feature; in transverse section, on 

 account of the relative smallness of its tracheae (or tracheids) ; j.n 

 longitudinal section, on account of the loose spiral or annular 

 thickenings of their walls. The looseness of the spiral or annular 

 markings is due to the fact that these vascular cells are the first 

 formed constituents of the bundles, and that consequently they 

 are considerably stretched by the continuance, for a time, of the 



M.B. K 



