LIGHT AND VEGETATION 



67 



are found in plants shows the importance of intense 

 evaporation. 



Here, for example, is the composition of a crop of lucerne 

 from one acre, its total dry weight being 4-8 tons. 



All these elements, except those which are present in the 

 atmosphere, are derived from the soil and conveyed in the 

 water sucked up by the plant. Some are in the form of almost 

 insoluble combinations and can accumulate in the plant only 

 when a sufficient quantity of water, drawn from the soil 

 with traces of dissolved salts, has passed through it and 

 evaporated, leaving the elements behind. There is no doubt 

 that they are beneficial, as numerous investigations have 

 proved. 



Taking again the example of one acre of lucerne, it can 

 be calculated that transpiration displaces 1,920 tons of water. 

 Calcium, which is the metal found in the largest quantity in 

 the plant, must be contained in this water at a concentration 



246-4 0-6 



higher than , i.e. (approx.). 



4,300,800 10,000 



The proportion of these elements contained in the soil 

 varies with its composition and the roots are capable of 

 filtering to some extent those which may be too abundant and 

 preventing excessive quantities from penetrating into the 

 plant. But obviously it cannot receive more than is contained 

 in the water evaporated, so that if their presence is necessary 



