io6 



PLANT LIFE. 



The paired \ascular bundles within ihe stele occupy various 

 positions, and for i)urpose of location may be spoken of as 

 though single. If transverse sections of the stem are ob- 

 served, they may be seen either in a single row, roughly 

 parallel with the surface of the stem (fig. iii), or in se\"eral 

 concentric rows (fig. 122), or they maybe irregularly dis- 

 posed throughout it (fig. 112). No one method of arrange- 



Fig. 122. — Transverse section of tlie aerial stem of an onion {Anhnn ■Sr/ioeno/Tasn >n). 

 c, epidermis; ch, clilorophyll-bearing tissue ot cortex; r, colorless tissue of cortex; 

 £", i^' , vascular bundles (xylem bundles black, pliloem bundles dotted); xr, mecbanical 

 tissues connected into a cylinder; ///, pith; h , [tith canal formed by destruction of 

 cells. Magnified 30 diam. — After Saclis. 



nicnt is confine*.! to any of the lar^^^er groups of ].)lants, 

 although the first is characteristic of most dicotyledons, 

 while both the second and third methods are common among 

 the monocotyledons.''^ 



* So many exceptions are found to these last statements that it is best 

 not to indicate the arrangement of the humlles hv tlic terms dictHyle- 

 donous or rnonocolyledonous, as has been eommonlv done ; nor is it 

 possible to maintain the terms exogenous and endogenous, which have 

 long since become obsolete because misleading. 



