62 STUDY OF COMMON PLAKTS. 



VI. Some leaves exhibit remarkable "sleep move- 

 ments." What are these for? Cf. Darwin, Power of Move- 

 ment in Plants, Chap. VII. 



VII. Other leaves exhibit equally remarkable "hot sun 

 positions." Of what use are these to the plant ? Cf . Wilson, 

 Contributions from the Bot. Lab. Univ. of Pa., Vol. I, No. 1. 



Mechanical and Conducting System. 



The skeleton or framework of the leaf serves to support 

 the delicate green tissue, holding it so as to expose the 

 largest possible surface to the sun, and, at the same time, 

 giving the whole structure sufficient rigidity, strength, and 

 elasticity to resist mechanical violence. It also serves to 

 conduct a constant supply of water and mineral substances 

 to every part of the leaf, and to convey away elaborated 

 food materials. It is only by keeping these principles in 

 mind that an intelligent study of venation can be made. 

 Cf . Sachs, Physiology/ of Plants, pp. 48-53. 



I. Obtain the leaves of several ferns, e.g. Adiantum pe- 

 datum, Aspidium cristatum, Osmunda Claytoniana. Draw 

 an enlarged outline of a leaflet of one or more species, 

 showing the exact position of the veins. 



II. Compare the venation of a number of monocotyle- 

 dons, e.g. Tradescantia, Alisma, Sagittaria, Pontederia, 

 Calla, Arissema, Smilax. Draw accurately one or more 

 leaves. 



III. Examine the venation of the leaves of Catalpa, 

 Liriodendron, Fuchsia, and Nymphsea. How does it 

 compare from a mechanical standpoint with that of the 

 leaves previously studied ? 



IV. Study critically the structure of the leaf of a black 

 oak or red oak. Measure the widest space you can find 



