THE ROOT 157 
Magnesium is a constituent of chlorophyll and accompanies 
proteins. It is necessary for the formation of fats. Deficiency in 
this element results in chlorosis and poor fruit formation. 
Iron is essential to the formation of chlorophyll although it is 
not a component of that substance. When deprived of iron, 
green plants become pale and chlorotic, even though they are 
grown in light. 
While not absorbed as mineral salts, carbon, hydrogen and oxygen 
are likewise indispensable elements. The very life of all plants 
depends upon their availability. All three enter into the forma- 
tion of proteins, carbohydrates and protoplasm and the first two 
are essential to the formation of fats and oils. Water, which 
makes up the greatest part of protoplasm and forms the solvent 
for soil salts, consists of a combination of two parts of hydrogen to 
one part of oxygen. 
In determining which elements are essential for the normal 
growth and health of plants, it is customary to grow them in 
water solutions of various salts or in sterile sand or ground 
pumice containing a water solution of the salts. 
NON-ESSENTIAL ELEMENTS ABSORBED BY Root-Hairs.—These 
elements include those which are also absorbed as water-soluble 
salts but which are not required for the normal growth of all 
plants. In many specific instances, however, they may be 
beneficial. 
Silicon is an abundant element in many plants, notably the 
grasses, sedges and scouring rushes. It occurs in the ash of 
these plants as silicon dioxide (SiO:) and in most soils as silicic 
acid (H2SiO;). It is deposited mainly in cell walls of the periph- 
eral parts of stems, leaves and seeds and affords protection from 
penetration by the hyphz and parasitic fungi as well as from 
animal attack such as plant lice and scale insects. 
Chlorine, another inessential element, probably has an influ- 
ence on the translocation of carbohydrates from leaves to other 
organs under natural conditions. Most plants can attain com- 
plete development without it. The question of its rdle still 
remains unsettled. 
Aluminum is only occasionally found in plants. It is known to 
influence the color of flowers in Hydrangea hortensis. When 
