120 Science of Plant Life 



differ in (i) the kinds of tissue making up the bundles, and 

 (2) the arrangement of the bundles in the stem. We shall 

 first study the bundles and the arrangement in a dicot stem, 

 and then we shall learn how the stems of monocots and 

 conifers differ from those of dicots. 



The structure of a dicot stem. When a dicot stem is cut 

 across, the bundles are seen to be arranged in a ring. The 

 cylinder of tissue lying inside the bundle cylinder is the pith ; 

 outside the bundle cylinder is the cortex ; and covering the 

 cortex is an epidermis very similar to that of leaves (Fig. 69). 

 In older and harder stems the outer cortical cells have thick 

 walls and form a corky or hard outer covering that replaces 

 the epidermis. The pith and the inner part of the cortex 

 are made up of rounded, thin- walled cells called parenchyma. 

 In annuals and young perennials the cortical parenchyma con- 

 tains chlorophyll and resembles the mesophyll of the leaf in 

 appearance and function. It is this tissue that forms the 

 inner " green bark " of twigs and gives the green color to the 

 stems and branches of herbaceous plants. 



There are, then, four distinct layers in dicot stems : (i) on 

 the outside is the epidermis; (2) from the epidermis to the 

 bundles is the cortex; (3) inside the cortex is the hollow 

 bundle-cylinder ; (4) the pith forms the axis of the stem, fill- 

 ing the space inside the cylinder of bundles (Fig. 69) . 



Between the bundles of the dicot stem there are strands of 

 parenchyma cells that connect the pith parenchyma with the 

 cortical parenchyma. These are the pith rays. They con- 

 vey food across the stem, and with the other parenchyma 

 cells form a complex tissue system in which excess foods 

 accumulate and from which they later move to other parts of 

 the plant. 



