34 THE TISSUES OF PLANTS 



somewhat thicker, with less tapering ends and frequently 

 with less thickened walls which are more strongly ligni- 

 fied than those of bast fibers* 



Laboratory Studies, (a) Break the shell of a hickory nut, 

 almond, coconut, walnut, peach-stone, etc., and after smooth- 

 ing the broken surface, cut off a thin shaving, using a pocket 

 knife or scalpel held at rather an oblique angle. Mount in 

 water and a little potassium hydrate. The very small cell 

 cavities show connecting pits or canals radiating from them to 

 the original cell wall where they meet similar canals from the 

 centers of adjoining cells, being separated only by the thickness 

 of the original wall. Concentric markings are visible in the 

 cell walls in some cases. 



(6) Determine whether the walls in sc'erenchyma are made 

 of cellulose or are lignified, by testing one section with a 5 per 

 cent, aqueous phloroglucin solution followed by hydrochloric acid 

 which gives a red color for lignified walls, and another section 

 with iodine solution followed by somewhat diluted sulphuric 

 acid which gives a blue color for cellulose walls. 



(c) Sclerenchyma may be found and studied (1) as the little 

 "grit" bodies in the flesh of the pear or sapodilla (Achras 

 sapoia), (2) in the underground stem of the brake (Pteridium 

 aquilinum), (3) next to the epidermis in the prickly pear 

 (Opuntia), as well as (4) in coats of many seeds, e.g. apple, 

 squash, wild cucumber, and (5) forming the body of the 

 seeds of many palms, e.g. date. 



(d) Examine a young leaf-stalk of the squash or pumpkin 

 and note the whitish bands, 1 or 2 mm. wide, which extend from 

 end to end just beneath the epidermis. These are bands of 

 collenchyma. They may be readily torn out, when the stalk 

 will be found to have lost much of its strength. 



(e) Make a very thin cross-section of the leaf -stalk of one of 

 the foregoing plants, exactly at right angles to the axis of the 

 collenchyma strands, and examine under low and high magnifi- 

 cations. Test with iodine and sulphuric acid to determine the 

 composition of the walls. 



(/) Make longitudinal sections through these collenchyma 

 bands. If good sections are obtained the thickened angles 

 (becoming thin toward the point where the thin cross* walls 

 occur), chloroplasts and nuclei will be found. However, only 



