48 



THE VEGETATIVE FUNCTIONS OF PLANTS 



serve more of the water taken in than do other species. 

 The two extremes in this respect are desert plants, such 

 as cacti, sage-brush, and euphorbias, and water-loving 

 plants, such as water-lilies, ferns, touch-me-not or jewel- 

 weed, and cucurbits like pumpkin and squash. A con- 

 venient method of measuring these differences is to com- 

 pare the weight of water absorbed with the weight of 

 dry matter produced. This ratio is known as the relative 

 water requirement of the plant. Thus, if a given plant, 

 during its growth, has taken in loo pounds of water, and 

 the soHd matter produced, when dried out to a constant 

 weight in a drying oven, weighs 2 pounds, the relative 

 water requirement is (100 : 2) 50. 



44. Government Experiments. — In experiments con- 

 ducted for the United States Department of Agriculture, 

 for the purpose of ascertaining the relative water require- 

 ments of various plants, it was found that the weight of 

 water taken in by hubbard squash plants amounted in 

 some cases to over 6,000 times the weight of the fruit, 

 and to over 900 times the weight of the total dry substance, 

 not including the roots. Other ratios, in round numbers, 

 were ascertained, as follows: 



Table I. — Water Requirements of Plants 



