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 (Trifolium), beech (Fagus), apple (Ma/us), 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 



O.f 



Fig. 41. Transverse section of leaf of Trifolium pratense, o. j, upper surface ; 

 j, under surface ; o, epidermis ; sp, stoma ; oca, crystals ; hlz, wood of fibro- 

 vascular bundles ; -, vessels ; ivb, phloem ; bf, bast fibers. After De Vries. 



fibrovascular bundles. What 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. Examine 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 



