CHAPTER XIV 

 STEM STRUCTURE 



Plant stems may or may not undergo the series of 

 changes incidental to the formation of secondary 

 tissues. The stems and rhizomes of monocotyledon- 

 ous plants usually retain the primary structures, with 

 but slight variation, throughout life. In dicotyledons 

 the primary structures undergo the changes consequent 

 to the formation of secondary tissues. Herbaceous 

 or annual stems exhibit primary structure. They 

 last but a single season, although their rhizomes or 

 roots may be biennial or perennial, giving origin to 

 a new stem each season. The woody stems of most 

 dicotyledonous plants are the result of the develop- 

 ment of secondary structures. 



PRIMARY STEM STRUCTURES 



Primary stem structures are present in the earlier 

 stages of stem development in both monocotyledons 

 and dicotyledons. The dermatogen, periblem and 

 plerom zones, to which reference has been made in 

 the section dealing with the origin of tissues, and 

 again in the section on root structures, are also pres- 

 ent in the rudimentary stem. The primary tissues 

 developed from these zones are in many respects 

 similar to the primary tissues of roots; but several 

 striking differences are apparent. The dermatogen 



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