STEM STRUCTURE 167 



Cortical Parenchyma. — The primary cortex of stems 

 is bounded on the outside by the hypodermis and on 

 the inside by the endodermis. The cortical cells 

 represent the original parenchymatic tissues of the 

 periblem region. The primary cortex is several layers 

 of cells in thickness, and various nutrients may be 

 stored within these cells. The individual " cells are 

 usually thin-walled and irregularly circular in out- 

 line when seen in transverse section. In longitudinal 

 view the cortical cells are either rectangular or polyg- 

 onal. In certain herbaceous stems, the cortical 

 cells become thick-walled through deposition of extra 

 amounts of cellulose upon their walls, and are thus 

 transformed into collenchynia. These groups of col- 

 lenchymatic cells serve as supporting tissues which, 

 because of the short life of such plants could not other- 

 wise be produced in sufficient amounts. Stone cells 

 are occasionally developed among the cortical cells. 



Endodermis. — In stems, as well as in roots, this 

 tissue separates the periblem tissues from those of 

 the plerom. It is the innermost of the primary periblem 

 tissues and its presence is an indication of primary 

 stem structure. The endodermis usually consists of 

 a single layer of cells and may not be as distinct as 

 the corresponding tissue in roots. The individual 

 cells are usually slightly colored and rectangular or 

 polygonal in outline when viewed in transverse section. 

 In certain roots the endoderm^l cell walls show uni- 

 form thickening on all sides; in other instances the 

 side of the cell toward the cortex remains thin-walled. 



In longitudinal section the endodermal cells resem- 

 ble the corresponding cells of the root, being long and 

 similar to fibers except that one wall is always thicker 



