STEUCTTJEE OP THE STEM. 



65 



Study the bundles in various portions of the section and notice particu- 

 larly whether the relative amount of surface in each covered by ducts and 

 by thick-walled wood-cells or sclerenchyma cells is everywhere the same. 



On the whole the structure of monoootyledonous stems is 

 much simpler than that of dicotyledonous stems. The bun- 

 dles which they contain are somewhat similar to those which 

 the exogenous or outside-growing stems of dicotyledons form 

 at a very early period of their growth. 



But while in exogens these bundles soon unite into a ring 

 of woody tissue, with a cambium layer outside, capable of 

 continual growth inward and 

 outward, in the endogenous 

 or inside-growing stems of 

 monocotyledons this is not 

 the case. True cambium is 

 not formed, but the procam- 

 bium which precedes the 

 mature bark-cells and wood- 

 cells is all transformed into 

 cells of bark or of wood, 

 which attain their full size 

 and are then incapable of 

 giving rise to new cells of 

 any kind. Therefore, the 

 stems of such perennials as 

 palms remain unchanged in 

 diameter year after year. 



Monocotyledonous stems which do increase in diameter from 

 year to year do so by the introduction of new bundles among 

 the old ones. This growth by interposition of new bundles 

 affords some justification for the name endogenous, often given 

 to the monocotyledonous stem. 



88. Distribution of Material in Monocotyledonous Stems. — 

 The well-known strength and lightness of the straw of our 



Fig. 54. — Cross-Section of Stem of Indian 



Corn, 

 c V, flbro-vasoular bundles ; g c, pithy ma- 

 terial between bundles. 



