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 
(Silphium laciniatum), ete., 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, ete. 
(f) 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. 
(g) 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. 
(hk) 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 
