212 EXPERIMENTS WITH PLANTS 



istic symptoms i. e., wilting, "drooping and yellowing of 

 the leaves, followed in time by the fall of the leaves, 

 beginning usually with the oldest. In regard to the 

 cause of the falling of leaves in autumn, see page 332. 

 It is interesting to note that when the leaves fall away 

 they leave a smooth scar ; this is due to the formation 

 of a layer of loose cells just at the base of the stalk (as 

 may be easily seen in a section under the microscope) . 

 If leaves of the Kentucky Coffee Tree, Tree of Heaven 

 (Ailanthus), Walnut or Ash are wrapped in a moist 

 cloth and placed in the dark, this layer forms very 

 rapidly (in the Coffee Tree within forty-eight hours), 

 with the result that the leaflets fall off, leaving a 

 clean scar. Is such a scar formed in most monocotyle- 

 donous plants, such as Corn, Grasses, Lily, etc.? 



Plants which are suffering from drought should be 

 watered, set in a cool, moist place and sprinkled with 

 water. If the plant has suffered severely it may be 

 necessary to remove some oi the older leaves. Cut- 

 flowers which have wilted may be placed in water and 

 covered with a wet towel, when they will quickly 

 revive. 



Leaves have various devices to protect themselves 

 against the wilting due to heat and dryness. (What 

 leaves remain fresh longest when removed from the 

 plant for purposes of decoration 1 Why ? ) Some 

 of these are obvious, and we may test them by 

 means of the apparatus shown in Fig. 119. Roll the 



