180 



PART II. THE INTIMATE STRUCTURE OF PLANTS. 



time they change their position somewhat, so that they come to lie 

 on the same radii as the bast-bundles, or the bast-bundles may also 

 deviate somewhat from their straight course, and thus the conjoint 

 collateral bundles come to be constituted. As a rule, these changes 

 of position are accompanied by an increase in number of the bun- 

 dles, 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 



f; s 



ft' 



FIG. 138. Radial longitudinal section of a conjoint, closed, collateral bundle 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; 

 o sieve-tubes of the bast, with companion-cells s ; spaa', the protoxylem; aa' remains of 

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

 conjunctive tissue. (Compare this with lig. 137.) 



in an isolated or a conjoint bundle) consists essentially of lignified 

 tracheal tissue (tracheae, or tracheids, p. 134), together with a 

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

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

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

 account of the relative small ness of its tracheae (or tracheids) ; in 



