218 



Practical Plant Biology. 



between these conducting tracts and those of the Pine is that they 

 do not possess a layer of cambium between the bast and the wood, 

 and hence these bundles do not grow in thickness. The dividing 

 cells of the procambial tracts differentiate completely into wood, 

 bast and fibrous bundle sheath leaving none over to develop as 

 cambium. Bundles or conducting tracts which like these have 

 no cambium are called " closed bundles " in contradistinction to 

 those which continue to grow in thickness, which are called " open ". 



m 



FIG. 75. Ranunculus repens, conducting tract of stem, transverse section, 

 x 100. b, bast; c, cortex: m, pith ; />#, protoxylem; r, medullary ray; 

 s, sheath ; wp, wood-parenchyma ; x, tracheae of wood. 



In general it will be found that trees and shrubs have " open " 

 bundles in their stems and roots, while plants with "closed" 

 bundles remain herbs. The vascular bundles of practically all 

 leaves are " closed ". 



The bundle-sheaths appear to discharge the functions of a 

 skeleton, i.e. they support and protect the soft tissues ; but their 

 exact r61e in these functions is not perhaps at first sight obvious. 

 The cells forming the sheath are long narrow spindle-shaped cells. 

 Their walls are very thick, so that the cavity of the cells is greatly 



