THE LEAF. 67 



II. Extent of leaf surface. Measure accurately the 

 superficial area of an average leaf of a geranium or 

 other common plant, and estimate its entire 

 leaf surface. 



III. Generic and specific characters drawn from leaves. 



IV. Variability. Compare the leaves of any individual 



plant, a rose bush, for example, and observe their 

 different forms. 



V. Leaves of insectivorous plants. Drosera rotundifolia 

 is widely distributed and is easily cultivated in the 

 laboratory. It is a most valuable plant for pro- 

 longed observation and experiment. Cf. Darwin, 

 Insectivorous Plants. 



REVIEW AND SUMMARY. 



The leaf is the most characteristic, and, in some respects, 

 the most important part of the plant. The venation and 

 various peculiarities of form and structure are . 



^ A cnaracter- 



usually sufficient to indicate at once the class, isticpartof 

 and not infrequently the genus or species to the P lantl 

 which a plant belongs. Even those who have had no 

 special botanical training readily distinguish the oak, 

 willow, maple, and various other plants by the leaf alone. 

 Hence in determining relationships special attention is 

 given to characters drawn from leaves, and it becomes 

 necessary to define these with care and precision. Physio- 

 logically, too, the leaf is engaged in work peculiar to 

 plants, work of a nature that cannot be performed by 

 animals, and upon which they are dependent for their 

 continued existence on the globe. A clear conception, 



