THE ROOT 65 
solution and iodine tests, that, while sugar passes 
readily, starch will not pass through a permeable 
membrane. This shows the purpose served at germin- 
ation by the conversion of starch into sugar. Inside 
the root hair there is a condition of things very similar 
to that existing inside the eggshell, for the hair 
contains a solution of sugar and other substances, 
which causes the less dense solution of minerals to 
pass inwards from the soil. 
There is another point which must be considered 
here. Whereas the apparatus we have used has no 
life, the root hair is a living cell and the protoplasm 
within exerts a more or less selective influence in 
connection with the absorption that takes place. 
Certain substances are not taken in at all, or are 
absorbed only in small quantities, while different 
plants, differing as to their requirements, take their 
food elements in different proportions from the soil. 
Tt is on this circumstance that the rotation of crops 
is based. 
Root hairs are very short lived, as a rule lasting 
only a few days. As the root tip is constantly pushed 
forward the hairs behind disappear and new ones are 
continually formed behind the growing point. This 
is an advantage to the plant in two ways. The hairs 
are constantly exploring newer and fresher soil areas, 
and are removed from contact with certain injuricus 
substances they themselves exerete or send out into 
the soil. 
In pulling up a plant from the earth, the root hairs, 
owing to their close adherence to the soil grains, are 
almost invariably broken off, and where, as in very 
loose soil, they may be got out uninjured, they will 
carry with them numerous particles of the soil itself 
(Fig. 43). This intimate contact between root hairs 
and soil grains is a great advantage to the plant, for 
it is the invisible film of water that surrounds each 
E 
