ILLUSTRATIVE STUDIES 45 



it can be determined how far back from the apex the onginal 

 cellulose walls first become modified for specific purposes. 



Notice where the first elements of the vascular bundle appear 

 in the procambium and how in older segments of stem these 

 have become longitudinally stretched; 



2. Make cross and longitudinal sections of Aristolochia stem 

 where the stem is older than where the sections in the above 

 studies were taken, but where the cambium has not yet begun 

 to add to the thickness of the stem. The stem in this region 

 will probably be too hard for the paraffin process and the sec- 

 tions may be cut free-hand or on a sliding microtome as de- 

 scribed on page 262. Double stain the sections in erythrosin and 

 iodine green and make permanent mounts in balsam (page 263). 

 Note the changes which each tissue has undergone since the 

 earlier stages and make drawings to scale to show these changes. 

 Think over carefully what you have seen and embody your 

 results in your permanent note-book. 



If it is thought best not to make the permanent mounts as 

 above suggested the sections can be examined in chloroiodide 

 of zinc (page 292) in which cellulose walls will be blue and all 

 others yellow. One objection to this reagent is that it swells 

 the walls more or less. In its place aniline sulphate might be 

 used (page 287) and then the lignified walls would be yellow 

 and all others would be uncolored. Cutinized walls could 

 then be demonstrated by leaving sections for several hours in 

 alcanna tincture (page 285) when these walls would be pink. 



3. Note the functions that in this chapter are attributed to 

 each tissue, and see in what ways the tissues are adapted to 

 them. Enter your observations in your permanent note-book. 



4. Make a cross section of a leaf through one of the lateral 

 veins and identify there the epidermis and the vascular bundle 

 of the vein. The rest of the tissues belong to the fundamental 

 or ground tissue, called in the leaf the mesophyll. Since the 

 leaf is to be studied in detail in another place it will suffice here 

 to enter in the note-book a simple diagrammatic drawing showing 

 the relative positions and amounts of these different tissues. 



