176 



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 hast 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 

 are stereome and mestome. Stereome 

 means the mechanical tissues of a plant 

 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- i i 



J. ■ 11 n n. whole. 



pamon cells. B, the 

 end partition be- 



tween two sieve ves- g Rings in the Wood. AgeofTrecs.^ 



sels showmg by , . ■= j 



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

 pores which form t^g ^ut end of a freshly felled tree? 



the sieve. 



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 



Fig. 61. — Cells of the 

 phloem. A, the ad- 

 joining ends of two 

 sieve vessels. The 



