CHAPTER IX. 



STEM AND ROOT STRUCTURES. 



A. Dicotyledonous Stem. — For the study of stem structure 

 a Sunflower may first be examined. Cut off a vigorous stem 

 and leave the upper portion in methyl blue.^ 



Cut a portion of a stem across a short distance below the 

 tip and peel off a thin transparent strip, the epidermis ; this 

 takes with it the scabrous hairs with which it is clothed, show- 

 ing that they are out-growths of this layer. They disappear 

 lower down, leaving the stem rough with their bases. 



Within the epidermis lies the cortex. Stain the cut end 

 with iodine and wash off. About a third of the distance 

 from the epidermis to the centre, a ring of small blue dots ap- 

 pears. They are starch grains which iodine stains blue and 

 are stored within the innermost layer of the cortex, the 

 endodermis. In the stem the endodermis may generally be 

 recognized by the starch it contains and is here known as the 

 starch sheath. 



Surrounded by the endodermis lies the stele. It consists 

 of a mass of ground tissue or conjunctive tissue within 

 which lies a broken ring of tissue forming elliptical patches. 

 In a fresh stem they appear white toward the centre and 

 greenish without. These are the complex fibro-vascular 

 bundles. Iodine has stained the inner portion a reddish 

 brown. This region, the xylem, contains the wood vessels 

 and fibres. If the stem which was placed in methyl blue be 

 examined it will be seen that these cells have taken up the blue 

 since they serve for the upward path of the transpiration 

 current. 



^This stain does not injure the stem, a few grains should be dissolved 

 in water until the solution is a clear bright blue. 



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