The Food of Plants. 343 
is absolutely indispensable; that under certain circum- 
stances, soda may be eliminated without injury ; that iron is 
essential to the formation of chlorophyll ; that calcium per- 
forms a function somewhat similar to that of the potash; that 
itmay to some extent replace it, and that it is possibly con- 
nected with the formation of tissues; that chlorine, and in 
some cases, sulphuric acid, is essential to the proper trans- 
fer of the substances digested in the leaves, to the parts 
where required by growth ; that magnesium is an element 
of uncertain value in the internal physiological processes, 
but that it has a definite value in the soil, where it aids in 
tbe distribution, and thus in the more complete appropria- 
tion of potash; that silica cannot be eliminated without ma- 
terially affecting the strength of the plant, and that phos- 
phorus bears an important relation to the various processes 
of ripening in the fruit. 
Another very important lesson to be derived from such 
special cultures, especially when combined with chemical 
analysis, is the fact that plants exercise a selective power 
with reference to the food supply; that is to say, if a plant 
were grown in a solution containing exactly the same pro- 
portions of all the elements entering into its composition, it 
would be found not to absorb them all in the same quantity, 
but some would be used much more largely than others. 
This becomes more obvious if we inspect the composition 
of the ash of different plants, or even of the same plant 
under different conditions or at different stages of growth. 
It thus appears that some plants are special potash 
feeders, others use more lime, yet others an excess of’ soda, 
and this fact constitutes the foundation on which the well 
known system of rotation of crops is based. This briefly 
stated, is as follows :—When plants are grown continuously 
upon the same piece of land for a number of years, those 
elements upon which that particular class most largely 
feeds, will be withdrawn in excess of the ability of the soil 
and the natural chemical processes there taking place, to 
restore them. The soil is therefore said to suffer special 
exhaustion, because it is deficient in one or two elements 
