88 TEXTBOOK OF PLANT PHYSIOLOGY 



been called forth by the statement of Mitscherlich that the con- 

 stants of the action of the growth factors are perfectly equal in all 

 plants. The difference in the requirements of different plants 

 with regard to mineral substances having been repeatedly and 

 incontestably established, this statement seems improbable. 



30. The Use of Mineral Substances by Various Plants. 

 Absorption of Soil Nutrients at Different Stages of Growth.— The 

 chemical composition of the ash of plants is variable, depending 

 largely on the environmental conditions under which they are 

 grown, their general properties, and their age. We have already 

 noted that the dryer the climate and the greater the amount of 

 salts in the soil, the higher will be the ash content of a plant. One 

 cannot conclude from this, however, that the accumulation of ash 

 in the plant is merely a mechanical process — simply the result 

 of evaporation of water from the leaves, the salts dissolved in it 

 remaining in the plant. Plants growing close together on the same 

 soil always show differences in ash composition, thus indicating 

 clearly the selective accumulation of some elements in preference 

 to others. One of the reasons for such a selective accumulation 

 of mineral substances in the cell is the unequal permeability of the 

 absorbing root cells: they are readily permeable to water, but the 

 substances dissolved in the water enter with greater difficulty 

 (see Art. 37). Besides, the different ions penetrate with dif- 

 ferent facility into the cells, and from there pass into the other 

 parts of the plant. The other possible reasons for unequal accumu- 

 lation of different substances in the cells will be discussed later. 

 Similarly in various parts of the plant the composition of the 

 ash differs considerably. The following table contains data 

 on the composition of ash of the vegetative organs and seeds of 

 some important agricultural plants. As is customary, the per- 

 centages are calculated not on the basis of pure elements but as 

 their oxides, and the amount of each oxide is given as the per- 

 centage of the total weight of pure, carbon-dioxide free ash. 



The table shows that seeds are rich in phosphorus and potas- 

 sium, elements very necessary for the building of new organs and 

 very often lacking in the soil. The stems and leaves of cereals 

 often show a high silicon content, and high calcium content in 

 flax and clover. In the subterranean storage organs, large 

 amounts of potassium are present. This seems to suggest an 

 important role of this element in the storage of carbohydrates. 



