42 INTRODUCTION TO BOTANY 
can penetrate has received the special name of osmosis. Os- 
mosis is not quite so simple as diffusion, since the movement 
of particles of the liquids is a good deal affected by the nature 
of the partition. In Chapter IJ it was stated that soil water may 
be taken up by root hairs. It may now be seen how osmotic 
action affects this process of taking up water by root hairs. 
By osmosis soil water may pass through the root-hair walls 
into the interior of 
the root hair and 
thence to other root 
cells. Obviously, 
when the liquids of 
the soil are more 
dense than the liq- 
uids within the root 
hairs, the root hairs 
will lose some of 
their water to the 
soil outside, and if 
enough water is 
lost in this way, 
the contents of the 
A, in normal condition; B, the same root hairs after a b 
being treated with a solution of common salt root hair may be- 
come plasmolyzed. 
41. How food is carried through plants. Applying the prin- 
ciples of osmotic action to the case of a starch-loaded leaf, it 
is evident that, as fast as the starch grains temporarily de- 
posited in the chloroplasts of the leaf are changed into sugar, 
some of the sugar in the denser cell sap thus produced will 
pass on to the more watery sap of adjacent cells. From these 
cells, in turn, portions of sugar will pass on to still more dis- 
tant cells. In a similar way, when a potato tuber is planted 
and begins to sprout, the sugar formed from the reserve starch 
in the potato passes into the more watery sap contained in 
the sprouts. This sap is constantly losing sugar that is used 
as building material for the young growing stems and leaves, 
Fic 29. Root hairs 
