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 too) the xylem and 
phloem are separated by the cambium layer, which, through 
its divisions, causes the stem to increase in thickness each year. 
This type 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. 
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