86 CLASS-BOOK OF BOTANY. 
STRUCTURE OF ROOTS AND UNDERGROUND 
_ STEMS. 
These parts of plants may be conveniently studied in the 
field or garden, but it will be found practically difficult in 
many cases to introduce them into the class-room unless they 
have been previously cleaned for examination. Frequently 
also their structure and mode of growth can only be accurately 
made out im situ. 
The rootis the part which serves to attach the plant to the 
ground, and to absorb from the soil the liquid nutriment requi- 
site for its growth. Its direction of growth is quite different 
from that of the rest of the plant, as it always seeks to go 
downwards and to avoid the light. And this difference is 
manifested from the beginning of the plant’s life. If wesoaka 
bean in water, and then lay it on the surface of damp soil in 
a flower-pot, the first thing to protrude from the testa will be 
the radicle, and this will immediately proceed to bury itself in 
the soil; and this attempt will continue, in whatever direction 
we turn it. 
A root formed in this way, by the direct elongation of the 
radicle, is called a tap-root, and, necessarily, in all the plants 
we have hitherto considered, the first root is always a tap- 
root. Ina large number of plants—e.g., in most trees—this 
kind of root is persistent, and often attains a great size. A 
true root is never green-coloured, nor does it produce leaf- 
buds, however many 
branches or secondary 
roots may be developed. 
When these secondary 
roots are produced in 
creat abundance the 
root is fibrous. Ii it 
consists of a few tufted 
thickened fibres it 18 
termed fasciculated, 
and when the thicken- 
ing causes the main 
portion of the fibres to 
ARAN be greatly swollen they 
\ are said to be tuber- 
Fig. 171. Fasciculated tuberous root of ous, as in the Dahlia. 
Dahlia. In all such cases of 
thickening, in addition 
to the usual root-function, there has been acquired the secondary 
function of storing up food-materials, such as starch, sugar, 
oil, &e. This tendency in many plants has been taken advan- 
