ILLUSTRATIVE STUDIES 87 



still the wood fibers make their appearance and the wood zone 

 is continually broadened by the activity of the cambium as the 

 stem grows older. This stretches and breaks the coUenchyma, 

 and it, as well as the bast, becomes gradually of less and less 

 importance as the wood increases. 



Illustrative Studies 



I.. We have already seen collenchyma in both cross and lon- 

 gitudinal sections in the stem of Aristolochia ; we can, however, 

 find this tissue carried to a higher stage of development in the 

 stem of sunflower. Zinnia, hemp, and many other herba- 

 ceous plants: Make cross sections from such a'plant and mount 

 them in dilute glycerine. Note the great thickening of the 

 walls at the angles of the cells. The shapes of the cells, as out- 

 lined by the primary walls, before thickening began, can be 

 seen. Draw a few cells to scale. Measure the thickness of 

 the walls across the corners. Are not these thickenings really 

 vertical rods? Study longitudinal sections to see how con- 

 tinuous these rods are. 



2. In the stem of Aristolochia we found a sclerenchyma ring 

 made of long cells with walls hgnified and somewhat thickened. 

 These are bast-hke, but they are not typical bast fibers. Sun- 

 flower and species of Abutilon, flax and hemp will furnish good 

 examples of fibers of various lengths. 



Study cross sections of sunflower stem in aniline sulphate 

 (page 287). The bast fibers will be yellow. Or when the 

 fibers are clearly recognized they may be studied in dilute gly- 

 cerine. Draw a few cells to scale. Measure the thickness of 

 their walls. Macerate longitudinal sections in hydrochloric 

 acid-alcohol and ammonia (see under Maceration in Chapter 

 XVI) and tease out the fibers in a drop of dilute glycerine. Draw 

 a few fibers to scale and measure their lengths. 



Study cross sections of young and old stem segments of Abu- 

 tilon Avicennae mounted in aniline sulphate. Here the first- 

 formed groups of bast fibers belong to the pericycle and later 



