64 



Elementary Plant Pl.ysiology. 



56. Amount of stretching produced by imbibition 

 and turgidity. — Take two or three fresh and rapidly growing^ 

 nearly mature leaves of sunflower {Helianthus), maple {Acer), 

 Caialpa, or Plantago, and trace their exact outlines upon a 

 sheet of paper. Now place between folds of dry blotting paper, 

 and apply pressure in the same manner as in the preparation of 

 herbarium specimens. Change the papers daily, renewing them 

 with dried and warmed sheets. About a week later take the 



leaves from the 

 blotters, and lay 

 each one centrally 

 upon the print 

 made from it. 

 Trace the outUne 

 of the dried leaf. 

 In what portion of 

 the leaf has shrink- 

 age taken place ? 

 Compute the area 

 of the surface of 

 the fresh and 

 dried leaf. A 

 dried leaf will 

 show a shrinkage 

 of eleven to forty-five per cent, from the extension of the 

 fresh specimen. 



A growing leaf is under a state of tension from the 

 stretching force of the turgidity of all its living cells, and, 

 in addition, the walls are also slightly increased in both 

 length and thickness by imbibition. During desiccation as 

 practised above, the cells die, and turgidity is lost alto- 

 gether, while much of the water of imbibition of the walls 

 is lost, allowing the leaf to shrink from its original dimen- 

 sions. 



Fig. 40. — Shrinkage of leaves during drying. A, leaf 

 of Catalpa ; the inner figure shows outline of dry leaf. 

 B^ leaf of Ptaniago major ; the inner figure shows out- 

 line after drying. After Halsted. 



