46 



A TEXTBOOK OF BOTANY 



[Ch. Ill, 6 



darkness, cold, humidity, 

 and calm. This is assum- 

 ing an ample supply of 

 water in the soil, under 

 conditions for easy ab- 

 sorption, since otherwise, 

 of course, transpiration is 

 mechanically checked by 

 lack of available water. 



Thus it is evident that 

 transpiration is affected 

 by external influences in 

 preciseljr the same way as 

 evaporation, thereby rais- 



FiG. 20. — The Transpiro- 

 graph ; X 5- The plant, pre- 

 pared as shown by Fig. 19, is 

 adjusted on a balance in such 

 a way that when it has tran- 

 spired one gram of water, that side 

 of the balance rises and closes 

 an electric circuit. The current 

 acts on the electro-magnet (visi- 

 >le in the picture), which pushes 

 a pen against the 

 revolying time drum 

 (shown b>' the lines 

 and letters), and 

 simultaneously re- 

 leases from the ver- 

 tical tube a spherical 

 gram weight, which 

 runs through the 

 outlet tube on the right and 

 drops into the scale pan. The 

 latter is thus depressed, breaking 

 the circuit, which remains open 

 until another gram of water has 

 been lost. Compare the record 

 in Fig. 21. 



iSuch a precise and continu- 

 ously self-acting instrument is 

 t.vpical of those which it is the 

 aim of plant physiologists to pro- 

 vide for all of the plant processes. 



