THE ABSORPTION OF MINERAL ELEMENTS 91 



P2O5, 14 parts of CaO, and only 1.6 parts of Si0 2 . Those col- 

 lected in June showed already a markedly altered composition, 

 namely, 17 parts of K 2 0, 8 parts of P 2 5 , 42 parts of CaO, and 

 21 parts of Si0 2 ; and when gathered in October, before abscission, 

 K 2 made up 7 parts; phosphorus, 5; calcium, 51; and silicon, 30. 

 The less necessary elements seem to accumulate with age, while the 

 amount of potassium and phosphorus shows a relative reduction. 



This comparative decrease of potassium and phosphorus, how- 

 ever, does not indicate an absolute decrease of these elements. It 

 may result from a relative increase in the total quantity of other 

 elements. Analysis in which not the general amount of ash but a 

 definite number of leaves, for instance 1,000, served as the unit of 

 calculation, have proved that the amount of all the elements 

 gradually increases during the vegetative period. Shortly before 

 leaf fall a return movement may be observed in the leaves of the 

 more mobile elements, like K and Mg. The greatest part of the ash 

 content remains in these organs. It may again become available 

 to plants after the leaves have dropped and decayed in the soil. 



31. Plants with Special Nutrient Requirements. — Most plants 

 are capable of growing on different soils, for almost all contain 

 sufficient nutrient substances for their development. Hence, 

 climate permitting, it is possible to raise the same crop plants in 

 widely differing soil zones. Still not all soils can satisfy the require- 

 ments of different plants to the same degree, and the distribution 

 of the wild flora can often serve as an excellent indicator of soil 

 properties. A few characteristic plants may be considered as 

 definite indicators of certain soil conditions, not only of its chemical 

 composition but its physical character and water-holding capacity 

 as well. 



One of the most characteristic examples of such adaptation is 

 shown by plants growing in saline soils, called 'halophytes." 

 They are usually found on seashores, where the soil is heavily 

 impregnated with salt water, or in salt marshes and the alkali soils 

 common in regions of dry climate, where weathering proceeds 

 more rapidly than leaching, and soluble salts accumulate. Plants 

 of the usual type do not grow or grow poorly on such soil. They 

 are replaced by halophytes, which are distinguished by considera- 

 ble anatomical and physiological peculiarities. These plants often 

 possess fleshy stems and leaves, thereby resembling those of the 

 type of cacti (Fig. 38). This characteristic structure is hereditary, 



