SOIL AND CLIMATE. 23 



this case the water soaks into the earth in the channels or basins 

 only, but in soaking into the earth is distributed more or less evenly 

 throughout the subsoil of the plantation, and all that escapes being 

 taken up by the roots of plants rises again and evaporates over the 

 whole surface of the plantation, depositing any salts that it may 

 then contain on the surface of the soil as in the previous case. In 

 such conditions we often find date trees growing lustily in a soil the 

 surface of which is covered with a thick deposit of salt. This 

 has given the idea to some people that excess of salt does not 

 harm date trees of any age. The roots of the trees may, however, 

 be feeding in a stratum from which practically all the salt has 

 been removed and deposited on the surface of the soil where 

 it does not affect the plants. Very great damage may be 

 done to such a plantation by heavy rains or by flooding the land 

 artificially. The reason of this is that the salt deposit on the 

 soil surface is dissolved and a very strong solution of salt is 

 carried down to the roots of the palms. This solution may quite 

 easily be strong enough to be fatal even to vigorous well-grown 

 trees. 



It is believed that the harmful action of the salt on the palm 

 is chiefly one of " plasmolysis." What takes place in such a 

 case, may be roughly illustrated as follows : 



Tie a piece of permeable vegetable or animal membrane a 

 piece of animal bladder will do over the top of a glass thistle 

 funnel ; invert it ; fill some solution of salt and water into it, 

 and place the funnel, broad end down, in a vessel of pure water, 

 immersing the funnel till the level of the liquid inside it and that 

 in the outer vessel are the same. Shortly afterwards it will be 

 found that the liquid in the outer vessel tastes saltish and that 

 the level of the liquid in the funnel has become higher than that 

 of the outer vessel ; in fact that there has been an interchange 

 of liquids going on, and that more liquid has flowed from the 

 outer vessel into the funnel than in the reverse way (see illustration 

 No. 22, page -22a). The rates of flow of solutions through such 

 membranes have been measured by scientists and found to be 

 are no visible holes in the membrane separating 



