66 STUDY OF COMMON PLANTS. 



NOTE. The student should carefully consider what is taking place in 

 the cells of green leaves, inasmuch as a great deal of confusion has arisen 

 through lack of clear conception and expression. Since they respire like 

 other parts of the plant, leaves absorb oxygen and give off carbon dioxide 

 both day and night. On the other hand, as organs of assimilation, they 

 decompose carbon dioxide in the sunlight, giving off oxygen and employ- 

 ing the carbon in the production of starch. A complete discussion of the 

 subject would require much space, but the fundamental facts are as 

 stated above, and should be firmly fixed in mind. 



MODIFIED LEAVES. 



When some other function than that of assimilation 

 becomes predominant, leaves exhibit marked, and in some 

 cases extremely peculiar, modifications. 



I. Examine shoots of the common barberry. Determine 

 the morphology of the spines and give reasons. Compare 

 the spines of the common locust. Are they the same 

 morphologically as those of the barberry ? Examine dif- 

 ferent species of cacti and determine the morphology of 

 the parts. 



II. Study the tendrils of such of the following plants as 

 can be obtained and ascertain which of them are to be 

 classed as leaves or parts of leaves: Smilax rotundifolia, 

 Cobcea scandens, Adlumia cirrhosa, Echinocystis lobata, 

 grape-vine, pea, cucumber, etc. Note particularly any cases 

 in which only partial modification has taken place. Cf. 

 Darwin, Climbing Plants, Chaps. Ill, IV. 



III. Leaves of insectivorous plants. See Special 

 Studies. 



SPECIAL STUDIES. 



I. Correlation of the forms of leaves with their position 

 on the stem. See Lubbock, Flowers, Fruits, and 

 Leaves. 



