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STEMS 



vessels are found what are called companion cells. The 

 function of these companion cells is not understood. Phloem 

 also contains mechanical tissue in the form 

 of fibers. These bast fibers resemble the 

 wood fibers in appearance and in function. 

 Reference has already been made to 

 mechanical tissue (page 147). This ex- 

 pression refers to all tissues which serve 

 the plant merely for mechanical support. 

 It is now evident that the cells of a vas- 

 cular bundle are, as to function, of two 

 principal kinds mechanical and conduc- 

 tive. The fibers, both of wood and bast, 

 are principally mechanical in function, 

 while the sieve and tracheary vessels are 

 chiefly conductive in function. Two use- 

 ful words in connection with this topic 



FIG. 61. Cells of the 



phloem. A, the ad- are stereome and mestome. Stereome 

 joining ends of two means the mechanical tissues of a plant 



sieve vessels. The 



shaded cells which taken as a whole ; mestome means the 

 accompany them on conductive tissues of a plant taken as a 



the right are com- whole 

 panion cells. B, the 

 end partition be- 



tween two sieve ves- G Rings in the Wood. AgeofTrees. 



sels showing by 



shaded dots the (See Figure 58.) Have you ever noticed 



pores which form t h e cut end of a freshly felled tree? 



There before you lies the record of a 



history which has run through many years, perhaps through 



hundreds of years. To look at the rings in the wood, to 



notice carefully the beauty of the layering, to remember 



the generations upon generations of green leaves whose 



work in each season's sunlight went into the making of 



B 



