88 THE PLANT SKELETON 



groups have descended from the cambium. Find the evidence 

 for this. Draw a few cells from a mature group and others 

 from a younger group where thickening of the cell-walls is not 

 yet complete. Macerate longitudinal strips of the bark in hydro- 

 chloric acid-alcohol-ammonia, tease out some of the fibers in 

 dilute glycerine, and draw one or more to scale. Measure the 

 lengths of the fibers. 



Study cross sections of flax in aniline sulphate. The sec- 

 tions are best made from small lengths of stem imbedded in 

 celloidine or collodion (page 266). In your judgment what 

 is the origin of the numerous groups of fibers found here. From 

 the ground meristem of cortex? From the pericycle? From 

 the cambium ? The student should now be able to answer these 

 questions from his own observations. Make a diagram showing 

 the position and frequency of the groups. 



Macerate strips of bark as told above for Abutilon, only here 

 the strips should be 5 or 6 cm. long. Tease out a single fiber 

 and measure its length. Draw a portion of its length as seen 

 under high magnification. 



3. Study cross sections of seeds that' have been soaked in 

 water and imbedded in glycerine gum (see under this head in 

 Chapter XVI). Draw some of the cells to scale. Note the charac- 

 ter and frequency of pits in the walls. Treat some of the sec- 

 tions with phloroglucin (see under this head in Chapter XVI). 

 Are the walls of the stone cells lignified ? 



Make thin sections of stone cell tissue from the stone of a 

 peach and the shell of a cocoanut. This can be done by sawing 

 as thin a section as possible with a hack saw, and then rubbing 

 it to the requisite thinness between two water hones kept wet. 

 This is a slow process and one is in danger of losing the section 

 altogether toward the end of the operation; but sections ob- 

 tained in this way are worth the labor. Another way to get 

 sections that will do fairly well is to shave them off with a sharp 

 knife. They will curl tightly when made in this way, but the 

 thinnest may be selected and forcibly straightened out in a drop 

 of water. If they break in doing this, nevertheless the small 



