LABORATORY STUDIES 69 



permit the fall of the leaves and at the same time 

 covering the exposed surface with a cork layer which 

 prevents the loss of water or entry of harmful organisms. 



Laboratory Studies, (a) Examine the cross-section of a 

 very young twig of elder or of a young stem of lamb's quarters 

 (Chenopodium album) and note the supporting system which 

 at this stage consists of longitudinal strands of stretched 

 elastic collenchyma just under the epidermis. 



(6) In older parts of the stem of the same plant note how the 

 main supporting function has been assumed by the wood fibers 

 associated with the xylem of the vascular bundles and by 

 strands of bast fibers sometimes closely associated with the 

 phloem of the same bundles and sometimes independent of any 

 bundles. 



(c) Make a cross-section of a leaf of beech or lily or other 

 plant and examine. The special nutritive palisade tissue is 

 present next to the upper epidermis. In the lower part of the 

 leaf note the "sponge" parenchyma with its large intercellular 

 spaces. The leaf of cottonwood (Populus sp.), compass plant 

 (Silphiutn laciniatum), etc., will show palisade tissues on both 

 sides. 



(d) Make a cross-section of a stem of a water lily (Castalia, 

 Nelumbo, etc.) or of a rush (Juncus) or of some other semi- 

 aquatic or aquatic plant. Note the large intercellular spaces. 

 Note also the rather small development of water -conducting 

 tissues. 



(e) For examples of tissues devoted to storage purposes 

 study sections of a tuber of potato, root of sweet potato, pith 

 of twig of apple or sassafras, seed of date, etc. 



(/) Make a cross-section of the stem of ivy (Hedera helix) 

 for gum canals lined with secretory cells. Similar canals in the 

 wood and leaves of Conifers (pines, spruces, etc.) contain 

 turpentine. 



(0) Make a cross-section of the leaf of St. John's wort 

 (Hypericum) or leaves or fruit of the orange or lemon (Citrus) 

 for secretory reservoirs ("glands") in the tissue. 



(h) Examine various flowers and study the location and 

 'structure of the nectaries. Extra-floral nectaries may be found 

 on leaves of various plants, e.g. some of the plums. Other 



