88 TUB natuhe and work of plants 



venient species. Measure from day to day and find 

 the total increase in length, and note the region of 

 greatest growth. In general, it will be found that 

 grasslike leaves grow at the base, while others 

 extend chiefly by the development of the terminal 

 portion. 



114. Wilting. — If the leaf of any rapidly growing 

 plant is taken off and laid in the sun for an horn-, 

 it may be seen that it becomes limp and is said to be 

 loilted. Compare with a fresh leaf. It is quite flexi- 

 ble, and the soft tissues between the ribs appear to 

 be shrunken. Hold an end of the leaf in either hand 

 and pull until it breaks in two parts. Repeat with 

 a fresh leaf. The wilted leaf is as strong in this 

 way as the fresh one. It has not lost any of its 

 mechanical tissues, and its limpness must be due to 

 the loss of water. The cells of fresh leaves of the 

 plant are filled with water to such extent that 

 they are stretched and the walls are very firm, in 

 the same manner that the string of a bow is as rigid 

 as a bar of iron when the bow is prepared for use, 

 but quite limp and flexible when separate. 



At noonday in midsummer and at other times the 

 leaves do not receive as much water as they evapo- 



