LEAVES 



197 



evaporation from the pot and soil by a rubber cloth (Fig. 

 160) or some other device. Moisture will be seen to collect 

 on the glass and even to trickle down the sides. Some meas- 

 urements of the loss of water by transpiration are as 

 follows : a single stalk of corn lost four gallons of water in 

 173 days (the duration of 

 its life) ; a single hemp 

 plant lost nearly eight gal- 

 lons in 140 days ; a sun- 

 flower, whose leaf surface 

 was approximately nine 

 square yards, gave off 

 nearly one quart of water 

 in a single day. If such 

 measurements be applied 

 in a general way to tho 

 enormous area of leaf ex- 

 posure in a forest, or to any 

 great mass of vegetation, 

 some vague idea may be 

 obtained as to the enor- 

 mous volume of water vapor 

 that is being given off by 

 plants. 



Transpiration has been 

 regarded usually as a men- 

 ace to the plant, without 

 any corresponding advan- 

 tage. It was known that 

 wate,r must be supplied 



freely to the working cells to keep them in working condition, 

 as well as to supply to green cells the water used in photo- 

 synthesis, and it was realized that loss of water by evapo- 

 ration is inevitable. It has been demonstrated recently that 



transpiration isnot so much a danger to the plant that must 

 14 



FIG. 161. A broad-leaved plant, showing 

 the general horizontal plane of the leaf- 

 blades. 



