STEM STRUCTURE 43 



i). Lengthwise sections of the stem of squash or cucumber plants {sieve 

 cells or soft bast). 



E. Thin cross sections of young twigs of pine or oak, collected and pre- 

 served in late summer {cambium). 



F. Thin cross sections and lengthwise sections of apple, plum, maple, or 

 box-elder wood. Test with phloroglucin {wood fibers).'^ 



G. Thin lengthwise sections of any coniferous wood. Test with phloro- 

 glucin {tracheids). 



H. Thin lengthwise sections of the stem of castor-oil plant {Ricinus) or 



of banana fruit stalks {vessels). 

 I. Thin lengthwise radial sections of sycamore, sassafras, or red-cedar 



wood {wood parenchyma). 

 J. Thin sections of pith of the stem of elder or sunflower {pith cells). 

 References. Strasburger-Hillhouse, 6 ; Strasburger, Noll, Schenck, 



Karsten, 1. 



30. Comparative structure of monocotyledonous and dicotyledonous bundles.* * 

 Examine with a power of about 150 diameters : 



A. The cross section of a bundle of the corn stem stained with phloro- 

 glucin. 



B. The cross section of a bundle of Aristolochia stem, 2 stained with 

 phloroglucin. 



Decide by referring to your drawings in Sees. 25, 28, which is the outer 

 part of each bundle. Observe the number and position of the area made up 

 of lignified fibers (stained by the phloroglucin), the cambium (in B), and the 

 sieve tubes. These tubes are less easy to identify than most of the other 

 elements of the bundles, but may be known by their location : in A, partly 

 between but mostly outward (toward the rind) from the pair of large 

 vessels ; in B, just outside the cambium of the bundle. Note the general 

 resemblance between the two kinds of bundles, with the presence of cambium 

 in B as much the most important point of difference between them. 



31. The dicotyledonous stem, thickened by secondary growth. 



A. Cut off, as smoothly as possible, a small branch of hickory 

 and one of white oak above and belo^v each of the rings of 

 scars already mentioned, and count the rings of wood above 

 and below each ring of scars. How do the numbers corre- 

 spond ? What does this indicate ? 



1 Both hard-bast fibers and wood fibers are known as sclerenchyma, but they 

 differ somewhat in appearance and much in location. 



2 xhis section should be made from a young stem collected and preserved dur- 

 ing the early part of the summer. 



