CHAPTER XII 



THE EFFECT OF WATER UPON GROWTH 



Wb have already, in Chapter X, laid stress upon the impor- 

 tance of the imbibition of water for the growth of both plants 

 and animals. Here we may consider more in detail the rela- 

 tion between growth and water, both as concerns the rate or 

 quantity of growth and the direction of growth, or hydro- 

 tropism. 



§ 1. Effect of Water upon the Rate and Quantity 



OF Growth 



It naturally follows from what we know of the importance 

 of water for growth, that the rate of growth will be closely 

 dependent upon water supply. And as all growth-phenomena 

 have been better studied in plants than in animals, our further 

 illustration of this fact will be drawn chiefly from the former. 



First, plant germination demands a certain minimal quantity 

 of water. What this quantity is may be determined either by 

 finding the least amount which will permit of normal germina- 

 tion, or by measuring the amount absorbed by different seeds 

 before protruding their radicles. This latter quantity has 

 been shown by the careful determinations of Hoffmann ('65, 

 p. 52) to vary from between 40% and 60% of the original dry 

 weight, in the case of various cultivated grains, to over 100% 

 in the case of various Leguminosse. 



In fungi, likewise, Lbsage ('95, p. 311) has found that 

 there is a hygrometric limit below which Penicillium spores 

 will not germinate ; and that the interval elapsing before ger- 

 mination begins is the shorter the moister the atmosphere. 



The method employed by Lesage is of wide applicability. The tension 

 of the water-vapor formed above a saline solution is less than that formed 

 above distilled water ; and it diminishes in proportion to the concentration 



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