42 STUDY OF COMMON PLANTS. 



tions, and in the same way compare the cells of 

 the cortical parenchyma as seen in both. 



2. Taking the inner bark next, the sieve-tubes are 



easily recognized by their narrowness and length, 

 and also by their soft, light-colored walls, while 

 the bast parenchyma consists of much shorter and 

 wider cells. The medullary rays present a marked 

 appearance, looking, on radial sections, like brick 

 work. 



3. Look for crystals of calcic oxalate, often found in con- 



siderable numbers in cells adjacent to the sieve- 

 tubes. 



4. The bast fibers are to be looked for in places corre- 



sponding to their position in the transverse section. 



They'may or may not be found in some of the lon- 

 - gitudinal sections. Why ? 

 When you have found them, note the points-in which 



they differ from all the other elements of the bark. 



5. Passing to the wood, the large pitted vessels are at 



once recognized. It is seen that they are com- 

 posed of long cylindrical cells placed end to end, 

 their dividing walls having been absorbed, or with 

 only traces of them remaining, so that they form 

 continuous ducts. The wood fibers also are 

 greatly elongated, but are much narrower. Their 

 walls are very thick and the ends tapering, fitting 

 to each other so as to make a very compact and 

 solid tissue. 

 Notice whether the medullary rays present the same 

 appearance in the wood as in the bark. Test the 

 contents with iodine solution. Cells resembling 

 those of the medullary rays, but extending length- 

 wise of the stem, will be found. These constitute 

 the wood parenchyma. 



