298 HISTOLOGY OF MEDICINAL PLANTS 



BI-COLLATERAL VASCULAR BUNDLES 



In the bi-collateral vascular bundle (Plate 127) the xylem is 

 in between two groups of phloem — namely, an inner group and 

 an outer group. 



In pumpkin stem a bundle occurs in each angle of the stem. 

 The entire bundle is surrounded by parenchyma cells. 



In an individual bundle the xylem consists of large circular 

 vessels and a phloem containing large sieve cells, many of 

 which show the yellow porous sieve plates. 



OPEN COLLATERAL VASCULAR BUNDLES 



In the open collateral bundle (Plate 100) the xylem and 

 phloem are separated by the cambium layer, which, through 

 its divisions, causes the stem to increase in thickness each year. 

 This t>T)e of bundle is characteristic of the stems and roots of 

 dicotyledonous plants. 



The bi-collateral bundle occurs in many leaves. The xylem 

 in such cases is central, the phloem strands occupying upper 

 and lower peripheral positions. 



