76 



ELEMENTARY BOTANY. 



ply the loss. If a hole be bored in the trunk of a vigorous 

 young tree and into it one end of a glass tube be fitted air-tight, 

 while the other end dips down into a vessel of 

 water, the latter will ascend in the tube to a 

 varying height. If the experiment be per- 

 formed in the spring before the extensive 

 evaporating surface (leaves) appears, an oppo- 

 site effect would be observed. The flow of 

 sap from Maple trees in spring is referable to 

 the quick alternate contraction and expansion 

 of the contained air, 

 - ■ -.--. LL i ^ - i ^ - _ -L ^ ^ , J and perhaps other 



gases, due to the sud- 

 den changes of temperature, rather than to influ- 

 y ences connected with transpiration. 



Determine the amount of transpiration in a given time 



by using a plant growing in a small pot. Cover the pot 



Fig. 114. closely and completely with paper to prevent loss of water 



except through the plant itself, and place it on one of the 



pans of delicate scales. Balance with weights ; after an interval of time 



(a few hours at least), remove weights necessary to restore the balance. The 



weights removed will indicate the amount of evaporation. 



5. The water lost by transpiration is replaced by a con- 

 stant stream passing from the soil through the roots and 

 stems to the leaves. This current is intercepted when a shoot 

 is cut off and its leaves consequently soon wilt. If the stem 

 be placed in water so as to obtain a partial supply, wilting is 

 much delayed. Or if water is forced into the stem the par- 

 tially wilted leaves will regain their turgidity. In the com- 

 mon plants the upward stream of water is conducted exclu- 

 sively through the lignified cell walls. The rapidity of the 

 ascent is dependent on the rate of exhalation — perhaps be- 

 tween five and fifty inches per hour would be the commonest 

 rate. 



Use the apparatus described in the experiment under paragraph 3 (the U- 

 tube, with diameter one-half inch or less), as shown in Fig. 114, but dis- 

 card the horizontal tube and one stopper. In the other stopper closely adjust 



