THE STUDY OF BUNDLES 



289 



by the presence of a growing layer at the line of union of the 

 phloem and xylem. This layer of growing cells is called cam- 

 bium. Dicotyledonous stems have open collateral bundles. 

 (Fig. 455.) Closed bundles are those that cease growing very 

 early and have no cambium or growing layer. They are called 

 closed, perhaps from the fact that there is no means by which 

 they may become larger. Stems of monocotyledons have 

 bundles of the closed collateral type. Examine with high 

 power cross -sec- 

 tions of menisper- 

 mum stems and 

 corn stems (see 

 Figs. 454, 455, 457), 

 that have been 

 stained with hema- 

 toxylin. Study the 

 tissues found in a 

 single bundle of 

 each, with the aid 

 of the illustrations. 



491. In concen- 

 tric bundles, the 

 xylem is centrally 

 placed in the 

 bundle and the 



phloem is all around it, as in club mosses and ferns (Fig. 

 456) ; or the phloem is in the center of the bundle and the 

 xylem surrounds it, as in the underground stems of some 

 monocotyledons, as asparagus. (Figs. 458, 459.) 



492. To see concentric bundles: Prepare cross-sections 

 of the stem of pteris or aspidium. They should be cut very 

 thin and stained with hematoxylin. Make a sketch showing 

 the arrangement of bundles. Bicollateral bundles differ 

 from the collateral in having additional phloem on the inner 

 side of the xylem strand; as in pumpkins and squashes. 



459. Enlargement of a single concentric 

 bundle from Fig. 458. 



