MAIZE. STEM. 109 



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 

 annular thickening : in old stems it is partially 

 surrounded 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 spiral vessel : the remaining 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-shaped xylem, and is easily recognised 

 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 are Sieve- 

 tubes. 



v. Smaller cells (cambiform) between the sieve- 

 tubes. 



Surrounding the above tissues of the xylem and 

 phloem is a Sheath of sclerenchyma. Transitional 

 forms may be found on its internal side, between 

 sclerenchyma, and certain of the constituents of the 

 bundle. 



