STRUCTURE OF MONOCOTYLEDONOUS .STEMS 189 



they do not collapse under the pressure of surrounding tissues, 

 may form rings as in annular vessels, spirals as in spiral vessels, 

 or be more generally distributed over the wall, leaving only 

 small unthickened areas which constitute the pits characteristic 



Fig. 167. — Cross section of a vascular bundle of Corn highly magnified, 

 s, strengthening tissue; j), phloem consisting of sieve vessels (e) and companion 

 cells (c); X, xylem consisting of annular vessel (a), spiral vessel (h) and pitted 

 (t); b, parenchyma cells. 



of pitted vessels. The xylem vessels are free from protoplasm 

 and are composed of cells joined in series with end walls resorbed. 

 They are known as tracheae, and are quite tube-like in struc- 

 ture and function. Through them the water and mineral salts 

 from the roots are carried, some reaching the leaves and buds 

 while much leaks out through the cellulose portions of the walls 

 to supply the tissues of the stem. Around the vessels are the 

 thin-walled parenchyma cells which may function some in con- 

 duction. 



In the phloem there are sieve vessels and companion cells. The 

 sieve vessels are composed of cells joined in series and so named 

 because of the perforated areas occurring in their end and side 



