236 HOW CROPS GROW. 
cap to protect the true termination or living point of the 
root in its act of penetrating the soil, Fig. 36 represents 
a magnified section of part of a 
barley root, showing the loose 
I cells which slough off from the tip. 
}/ These cells are filled with air in- 
stead of sap. g 
A most strik- 
ing  illustra- 
tion of the 
root -cap is 
furnished by 
the air-roots 
of the  so- 
called Screw 
Pine, (Panda- 
nus odoratis- 
simus,) exhibited in natural dimen- 
sions, in fig. 37. These air-roots issue 
from the stem above the ground, and, 
growing downwards, enter the soil, 
and become roots in the ordinary sense. 
When fresh, the diameter of the 
root is quite uniform, but the parts 
above the root-cap shrink on drying, 
while the root-cap itself retains nearly 
its original dimensions, and thus 
reveals its different structure. 
Distinction between Root and 
Stem.—Not all the subterranean parts 
of the plant are roots in a proper Fig. 37. 
sense, although commonly spoken of as such. The tubers 
of the potato and artichoke, and the fleshy horizontal parts 
of the sweet-flag and pepper-root, are merely underground 
stems, of which many varieties exist. 
' These and all other stems are easily distinguished from 
