CHAPTER VIII. 



THE EFFECT OF MOISTURE AND OF SALINITY. 



Effect of humidity of soil on plant growth Effect of dry and moist 

 surroundings on characters of plants Desert plants and Aquatic 

 plants Effect of moisture on Lepidoptera and on Molluscs 

 Characters of maritime plants probably due to saline environment 

 Conversion of A. saUna into A. milhausenii and into BrancMpus 

 Effect of increased salinity on characters of the cockle Influence 

 of salinity on rate of growth of Tubularians, and on size of sea- 

 urchin larvae. 



IN that the presence of water is absolutely necessary 

 to enable living organisms to exhibit activity, and very 

 probably, indeed, to enable them to retain vitality at all 

 for even spores contain a small percentage of water 

 so we should conclude that differences in the amount of 

 water in the environment of the organisms would form 

 a fertile source of variation. Such is, in fact, the case 

 in the Vegetable Kingdom. For example, the amount 

 of water in the soil has a considerable influence on the 

 rate of growth, as is shown in the table given below. 

 These figures, which were obtained by Hellriegel,* 

 represent the amount of dry substance contained in the 

 grain and chaff of barley which had been reared in soils 

 containing various percentages of the saturation quan- 

 tity of moisture. We see that the rate of growth 

 varies but little until the humidity falls below 30 per 

 cent., and then it diminishes so rapidly that with a hu- 

 * Quoted from Davenport's " Experimental Morphology," p. 353. 



