62 BOTANY 
as constantly replaced by fresh portions supplied from 
the meristem behind it. In pulling plants from the 
soil, the root-cap is almost sure to be broken from the 
tip of every root branch, but there are circumstances 
under which this structure can be seen. Some members 
of the screw-pine family, to which the kie-kie velongs, 
send out from their upper parts, roots which pass 
downwards through the air to the earth below. On 
the tips of these the root-cap is quite a conspicuous 
object. The kie-kie itself does not produce these prop- 
roots. It may also be observed without a microscope 
on the roots of Wandering Jew grown in a jar of 
water. The root-cap fits over the growing point just 
as a thimble fits over and protects the finger. 
Root Hairs. Having followed the root into the soil 
and seen how admirably it is suited for its work of 
boring, let us now examine the structures by which it 
is enabled to take advantage of the position it has 
attained, that is, see by what means it is able to 
eather for the use of the plant the water and minerals 
with which it has placed itself in contact. This brings 
us to the root hairs, which, just behind the growing 
points of roots and root branches, form a dense furry 
mass almost as compact as the pile on velvet. The 
mustard seedlings as shown in Fig. 40 with the root 
hairs displayed against the dark background of the 
cloth, present an appearance of wonderful beauty, and 
at the same time strikingly exhibit Nature’s prodigality 
in providing for the needs of her children. 
Roughly speaking, each hair is a little closed tube 
with very thin walls, so thin, indeed, that, by a process 
called osmosis, the water containing dissolved minerals 
is, under certain conditions, able to pass through from 
soil to plant, and certain waste products of the plant 
are able to pass outwards to the soil. Wherever two 
liquids or gases of different density are separated by 
a permeable membrane osmosis takes place. At present 
