The Structure of Stems 



131 



Fig. 74. Photograph of a cross-section of a young sunflower stem, 

 showing arrangement of the bundles. Locate the several tissues. 



palms; and (3) dicotyledonous plants (dicots), or plants with 

 net-veined leaves, like oaks, maples, sunflowers, asters, and 

 clovers. 



The stems of the plants belonging to these three groups differ 

 in (i) the kinds of tissues and cells 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 young dicot stem is cut 

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

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

 bundles is the cortex; and covering the cortex is an epidermis 

 very similar to that of leaves. In older and harder stems the 

 epidermis disappears and the outer cortical cells may be replaced 



