Transpiration and Water Movement 107 



and noting the strands. Examined microscopically a 

 bundle exhibits in cross section the typical collateral 

 arrangement. In this the phloem or sieve-tube part 

 is outermost, the xylem therefore within, and both are gen- 

 erally inclosed by a sheath of stereome or mechanical 

 supporting tissue. There is, however, no meristem or 

 cambium within the bundle, signifying a closed type. 

 The irregular distribution of bundles in the stem usually 

 precludes the formation of a ring of wood, and there is, 

 moreover, no bark in the usual sense. These are matters 

 of much physiological significance, both from the stand- 

 point of growth and conduction. 



In dicotylous plants the primary bundles are arranged 

 in a ring and are also commonly collateral. The inter- 

 position of secondary bundles (as subsequently discussed, 

 section 187) may result in the formation of a complete 

 wood-ring. Where no wood-ring is produced, the bundles 

 run parallel throughout much of the internode (Fig. 34), 

 divide and unite in a characteristic fashion at or near all 

 of the nodes, also sending off branches to the leaves at 

 each node. 



When a wood-ring is produced in a dicotylous stem, the 

 meristem of the bundles forms a continuous growing layer, 

 the outer portion of the wood-ring then consists of phloem 

 and the inner portion of xylem. The cambium between 

 permits the addition of seasonal or growth rings of phloem 

 and xylem on the outer and inner sides respectively. In 

 plants which attain a considerable age the parenchyma 

 accompanying the bundles loses its protoplasm and the 

 bundles cease entirely to take part in conduction. The 

 number of rings of new wood which may be active in con- 

 duction varies greatly with different plants. 



