MAIZE STEM 163 



of these the two smaller are developed first. Compare 

 sections of young stems. 



In many Monocotyledons the arrangement of the constituents 

 of the xylem in the form of a V is much more plain than here. 

 e.g. Asparagus. In other cases (e.g. Calamus} this arrangement is 

 not to be seen. 



The vessel nearest the centre of the stem has usually 

 annular thickening : in old stems it is partially sur- 

 rounded by an intercellular space, while the rings often 

 become detached, in which case the vessel is not easily 

 seen in transverse sections. Next this is a vessel 

 which has commonly a spiral thickening : the remain- 

 ing two have thinner walls with pitted marking, and 

 large cavity. 



Surrounding the pitted vessels, and between them, 

 are 



ii. A number of tracheides with pitted lignified 

 walls, and no cell-contents. Surrounding the inter- 

 cellular space above described is 



iii. A group of parenchymatous cells with thin 

 cellulose walls. These may be regarded as xylem 

 parenchyma. 



The phloem portion of the bundle lies between the 

 limbs of the V~ sna P e( i xylem, and is easily recognized 

 by the thin cellulose walls characteristic of soft bast. 

 It consists of 



iv. Elements with large cavities, in which transverse 

 septa (sieve-plates) often occur : these elements are 

 the sieve-tubes. 



v. Smaller cells (cambiform) between the sieve- 

 tubes. 



M 2 



