172 PLANT PHYSIOLOGY 



8. ELIMINATION. 



Having studied the modes by which substances are absorbed 

 into and transported through the plant, we have next to consider 

 the manner of their elimination from the plant. Of this there 

 are three phases, viz., (a) the elimination of water as vapor, or 

 Transpiration, the most striking of all; (b) the elimination of 

 liquid water, or Guttation; (c) the elimination of waste matters, 

 or Excretion. 



(a) Transpiration. 



It is a sufficiently familiar fact that ordinary plants give off 

 large quantities of water-vapor through their leaves, though 

 if any further evidence thereof is needed, it can be supplied by 

 simple experiments described in a note (under Demonstrations) 

 below. We may therefore proceed directly to the obvious first 

 problem of our study, which must be this :. 



In what quantities is water transpired from ordinary plants 

 under usual conditions? 



This may best be determined by the delicate quantitative test of 

 weighing, the utilization of which requires that the transpiration shall 

 not be complicated with evaporation from the soil. 



Experiment. Select a convenient single-stemmed potted plant, 

 preferably from the materials mentioned below, and cover pot and 

 soil with vapor-proof coverings, so that all loss of water must be 

 through the leaves and stem. Weigh the plant on a good balance 

 at intervals as frequent as practicable, watering the plant and renew- 

 ing the air inside the coverings once a day. Tabulate the loss through 

 two or three days. 



Precautions. While not essential, it is desirable for the better health 

 of the roots, and hence of the whole plant, to use a method for enwrapping 

 the pot such as will permit a daily renewal of the air. In watering it is 

 a good plan to start by giving the plant only about half its usual amount 

 (since all evaporation will be prevented), and thereafter to add each day 

 approximately the amount it has lost in the preceding twenty-four hours. 

 The weighings should be so arranged as to include the eight hours of greatest 

 daylight (thus being made at 8 a.m. and 4 p.m. or thereabouts), which per- 

 mits a comparison with the loss by night when the sixteen hours thereof 

 is divided by two. 



Materials. These have been studied thoroughly from the experi- 

 mental point of view by Miss Grace Clapp, with results contained in the 



