THE WORK OF LEA YES 213 



leaf or fold it two or three times, after the fashion of 

 a fan, and secure it by a rubber band. How does this 

 affect the rate of transpiration ? Do you find devices of 

 this kind among plants? Many Grasses (e. g., Corn) roll 

 up their leaves in the heat of the day or fold the 

 stomatic surfaces together so as to diminish transpi- 

 ration. Young leaves are far more easily injured by 

 drying agencies than old ; they are nearly always rolled 

 or folded (as they issue from the bud) in a manner 

 that greatly reduces evaporation, and it will be noted 

 that if one surface is more exposed than another it is 

 the stomatic surface which is protected. Make obser- 

 vations on this point: study especially the Tulip Tree, 

 Canna, fruit trees, Maple, Ferns, etc. (see Gray's Les- 

 sons, under vernation or aestivation). Many leaves are 

 permanently rolled (Heath plants, etc.). 



Again, we may cover the leaf in our apparatus with 

 cotton wool, pressed down tightly against the leaf by 

 means of wire netting laid over it and held in place by 

 clothes-pins. We may cover both sides of the leaf or 

 the stomatic surface only, and may note the rate of 

 evaporation in both sunlight and shade. Do you find 

 coverings analogous to this on leaves? Study the leaves 

 of the Dusty Miller, Horehound, Sage, Wormwood, 

 Mullein, Cinquefoil, etc. Many leaves have a hairy 

 covering du v ing their youthful, sensitive period, but 

 lose it as they grow older (Horse-chestnut, Beech, 

 Service -berry, White Poplar, Pear, etc.). 



