36 ANATOMY AND PHYSIOLOGY 



The bundle sheath surrounds tissues in which there 

 are three or four large open spaces: (i) The some- 

 what irregular space at one end is an intercellular 

 air passage; (2) the two on either side of the middle 

 are trachea. A ring from an annular tracheid may 

 often be seen projecting into the inner end of the air 

 passage; and one or more spiral tracheids may 

 lie between the two large tracheae. This group 

 of tracheae and tracheids constitutes the xylem, 

 or water-conducting tissue. It is woody in con- 

 sistency. The thin-walled, closely packed tissue, 

 at the opposite side of the bundles from the air 

 passage, is the phloem tissue, which under high 

 power can be seen to be composed of two kinds of 

 elements: (i) The larger cell-like spaces which are 

 cross-sections of the sieve-tubes; and (2) the very 

 small, nearly square cells, at the corners between the 

 sieve-tubes, which are the companion cells. The 

 phloem is concerned in the conducting of elaborated 

 food, perhaps chiefly the. nitrogenous foods. Com- 

 pare the vascular bundles near the periphery with 

 those at the center of the stem. What differences 

 are there in structure, and in the relation of one 

 bundle to another? How are the xylem and 

 phloem placed in the bundle in relation to the 

 periphery of the section? 



3. Examine a longitudinal section of a corn stem of 

 considerable length and make a diagram illustrating 

 the course of the vascular bundles, especially notic- 

 ing how and where the bundles (veins) from the 

 leaf enter the stem and the course they take. What 

 finally becomes of the leaf-bundles? 



