THE STEM. 



89 



The paired vascular strands within the stele occupy various positions, 

 and for purpose of location may be spoken of as though single. If trans- 

 verse sections of the stem are observed, they may be seen either in a sin- 

 gle row, roughly parallel with the surface of the stem (fig.' 72), or in 

 several concentric rows (fig. 79), or they may be irregularly disposed 

 throughout it (fig. 73). No one method of arrangement is confined to any 

 of the larger groups of plants, although the first is characteristic of most 



Fig. 79. —Transverse section of the aerial stem of an onion {Allium Schoenoprasuin). 

 e, epidermis ; ch, chlorophyll-bearing tissue of cortex; r, colorless tissue of cortex; 

 ^, ^, vascular bundles (wood bundles black, bast bundles dotted); sr, mechanical 

 tissues connected into a cylinder; 7n, pith; ky pith canal formed by destruction of 

 cells. Magnified 30 diam.— After Sachs. 



dicotyledons, while both the second and third methods are common 

 among the monocotyledons. But so many exceptions are found to these 

 last statements that it is best not to indicate the arrangement of the bun- 

 dles by the terms dicotyledonous or monocotyledonous, as has been com- 

 monly done ; nor is it possible to maintain the terms exogenous and en- 

 dogenous, which have long since become obsolete because misleading. 



110. Pith. — ^The pith is frequently found enormously 

 developed in those parts of the stem used for storing reserve 

 food, such as the tubers of the white potato and the yam. In 

 other plants, particularly those growing in water, it suffers 



