44 STUDY OF COMMON PLANTS. 



by strong bands of thick-walled cells, composing 

 the so-called bundle-sheath, which may be continu- 

 ous, or thinned out on the sides of the bundle. 



The bundle itself presents two parts for study : first, 

 the xylem, or wood, which includes the two conspic- 

 uous pitted vessels (recognized by their very large 

 openings), and the parts immediately adjacent ; 

 and second, the phloem, or bast portion, marked 

 by the peculiar appearance of its elements on 

 transverse section, its small cells being fitted in at 

 the angles between larger ones in such a way as 

 to give the effect of mosaic work.* 



Studying first the xylem, on both transverse and 

 longitudinal sections, we find that it consists of 



a. The large pitted vessels already noticed. Ex- 



amine their structure carefully, observing par- 

 ticularly the remains of the partition walls in 

 the form of heavy rings, indicating the origin 

 of the vessels in rows of cells placed end to 

 end. One or more smaller vessels lie between 

 them, and a little nearer the center of the 

 stem. One of these is conspicuously marked 

 by heavy thickenings in the form of rings, and 

 is called an annular vessel. Frequently the 

 surrounding tissue is absorbed, leaving only 

 the rings of the annular vessel to mark its 

 place. 



b. Thick-walled elements lying between the large 



pitted vessels. 



c. Elements with thinner walls surrounding the an- 



nular vessel. Some of these, as already stated, 

 have disappeared, leaving an irregular open 

 space. 



