Exchange and Movements of Gases and Liquids. 65 



57. Structure of a leaf of a mesophytic plant. — 



Examine the leaf of any convenient mesophytic species, such 

 as clover {Trifoliuni), beech [Fagtis), apple {Malus), oak 

 ( Quercus) . Place a few leaves in alcohol to extract the chlo- 

 rophyl, then trace the ramifications of the nerves. Make dia- 

 gram of same. Cut a thin cross-section of the petiole, and note 

 the formation of the bundles. Cut a thin cross-section of part 

 of the blade. Note the arrangement of cells contiguous to the 



Fi^. 4T. — Transverse section of leaf of Tri/olium praiense. o. J, upper surface ; 

 w J, under surface ; o^ epidermis ; j/i, stoma ; oca, crystals ; hlz, wood of fibro- 

 vascular bundles ; g^ vessels ; wb^ phloem ; b/, bast fibers. After De Vries. 



fibrovascular bundles. ^\'hat differences are to be found in the 

 arrangement of the cells on the upper and lower sides of the 

 leaf ? Take thin sections from the upper and lower surfaces. 

 Estimate the number of stomata per square centimeter in both 

 places, by the use of an eye-piece micrometer. 



58. Structure of a xerophytic leaf. — ILxamine the 

 surface, structure, and arrangement of the tissues of some spe- 

 cies adapted to living under arid conditions, or in a situation 

 in which transpiration is to be restricted. Russian thistle 

 5 



