TISSUE SYSTEMS 



347 



The number of bundles in a stele varies greatly. They are 

 usually very numerous in the stems of Monocotyledons {fig. T27), 

 few in those of Dicotyledons {fig. 726) and Cryptogams 

 {fig. 729), or there may be only one, as in the separate steles 

 of schizostelic, and the smaller steles of polystelic, stems. 



According to the mode of arrangement of the wood and bast. 



Fig. 732. 



Fig. 732. Collateral bundle from stem of a Jlonocotyledon. jiJi. Pliloom. 

 :r. Xylem vessels, p.jj/t. Protophloem. The bundle is surrouiifled by a 

 small-celled sheatli of sclerenchvma. Alter Kny. 



we have several types of vascular bundle. The two principal 

 ones are the collateral and the concentric. 



Collateral Bundles. — "We distinguish again two kinds of 

 these, one characteristic chiefly of the stems of Gymnosperms 

 and Dicotyledons, the other of those of Monocotyledons. 



In the first variety the bundle is roughly wedge-shaped, and 

 the bast is placed behind the wood {figs. 731, and 743, b). The 



