STEMS 



39 



1. Wood, on outside of old wood. 



2. Phloem, on inside of old phloem. 



3. Cork, on inside of old cork. 



4. Secondary cortex, on outside of old cortex. 



A two-year old woody dicot stem has the following general 

 structure: 



1. Bark, consisting of the following parts in order from 

 outside to inside: Cork, cork cambium, secondary cortex, 

 primary cortex, primary 

 phloem, secondary phloem. 



2. Cambium of vascular 

 bundle. 



3. Wood, consisting of 

 two layers, the youngest 

 toward the outside. 



4. Pith. 



5. Medullary rays, each 

 ray of several rows of 

 thin-walled ceils running 

 from the medulla to the 

 outer edge of the phloem. 



Monocot Stems.^The 

 corn stalk is an excellent 

 type of a monocot stem. 

 In this, as shown in cross-section of the stem in Fig. 18, 

 the vascular bundles (fibers) are scattered throughout the 

 ground tissue. They do not form a definite "vascular ring" 

 as in dicot stems. Moreover, the vascular bundles of most 

 monocots do not possess cambium, as in dicot stems. Hence, 

 new phloem and xylem are not produced each season, and 

 consequently there are no annual rings formed. Growth of 

 monocot stems results from (i) simple enlargement of cells 

 derived from primary meristem tissue, and in some instances 



Fig. 18. — Cross-section of cornstalk 

 stem; a, epidermis; b, cortex and peri- 

 cycle; c, ground tissue. {After Stevens.) 



