io6 



PLANT LIFE. 



The paired vascular bundles within the stele occupy various 

 positions, and for purpose 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. in), or in several 

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

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



Fig. 122. — Transverse section of the aerial stem of an onion {Allium Schoenoprasu7>i). 

 e, epidermis; c/i, chlorophyll-bearing tissue of cortex; r, colorless tissue of cortex; 

 g, g', vascular bundles (xylem bundles black, phloem bundles dotted); sr, mechanical 

 tissues connected into a cylinder; ;//, pith; /i, pith canal formed by destruction of 

 cells. Magnified 30 diam. — After Sachs. 



ment is confined to any of the larger groups of plants, 

 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 bundles by the terms dicotyle- 

 donous or monocotyledonous, as has been commonly done ; nor is it 

 possible to maintain the terms exogenous and endogenous, which have 

 long since become obsolete because misleading. 



