TISSUES AND SIMPLE ORGANS. 



85 



mechanical tissue bast of the vascular bundles) ; the conducting 

 elements must on the one hand carry the necessary water and the 

 various food-substances to the leaves, and on the other hand they 

 must conduct the products of assimilation from the leaves to the 

 stem. As we have already learned, the leptome in the stems of 

 monocotyledons and dicotyledons is placed toward the outer sur- 



FIG. 50. Vascular bundle of the stem of Zea Mays, (x 560.) 



v, Albumen-conducting tissue, leptome or sieve-tissue ; p, parenchyma ; s and r, primordial 

 (primary) vessels ; gr, secondary vessels ; I, intercellular space ; a, side facing outward ; i, 

 side facing inward. (After Sachs.) 



face, the vascular portion toward the interior; corresponding to 

 this arrangement we find the leptome of the leaves near the lower 

 surface and the hadrome (vascular portion) near the upper surface. 

 Occasionally skeleton-bundles (mechanical cells) which prevent the 

 tearing of leaves by strong winds, etc. , are brought into mechani- 

 cal operation ; these bundles are usually found near the middle of 



