THE STEM. 45 



The two sorts of elements that compose the phloem 

 are easily recognized on both transverse and 

 longitudinal sections. 



a. The sieve-tubes are large, with nearly or quite 



transparent contents, and here and there a per- 

 forated transverse septum looking like a sieve. 



b. The smaller cells placed at the angles of the 



sieve-tabes are the cambiform, or companion, 

 cells. Their thicker contents, smaller diame- 

 ter, and the absence of sieve-plates at once dis- 

 tinguish them from the preceding. 

 Having identified all the parts that have been 

 named, study them closely, and after you have 

 become perfectly familiar with the position and 

 structure of the different elements, draw and de- 

 scribe them. Meantime, look for any additional 

 features to which your attention has not thus far 

 been specially directed. See if you can recognize 

 the protophloem, a small group of rather indistinct 

 cells lying between the phloem and the bundle- 

 sheath. 



Study, too, more carefully, the structure of the sieve- 

 tubes. Try the effect of picric aniline blue on 

 these and other parts of the bundle. Apply 

 Schulze's solution to other sections, and phloro- 

 glucin (followed by hydrochloric acid) to still 

 others, and note the results. What parts of the 

 bundle are lignified? How about other parts of 

 the stem ? 



3. The fundamental tissue. Examine the large cells 

 composing the tissue, using both transverse and 

 longitudinal sections. Ascertain whether the large 

 cells of which it is made up present the same 



