106 PLANT STRUCTURES 
all sides at an angle to the main stem, these also 
tending in a mysterious way, if their course is dis- 
turbed by an obstacle, to resume their former direction 
of growth; the branches again divide, till at length a 
complicated root-mass is formed, sometimes of great 
extent, and capable of extracting water from a large 
volume of soil. Save for continued growth, the roots 
show little change in comparison with those exhibited 
by the aerial parts of plants; safely immersed in the 
soil, they heed not day or night, storm or calm, but 
steadily pursue their main function of supplying liquid 
food material to the green parts overhead. 
In many instances roots do not accomplish their 
work single-handed, but only in co-operation with 
certain lowly organisms; and these cases are so inter- 
esting and of so much economic importance that 
reference should be madetothem. The little swellings 
or tubercles upon the roots of Leguminous plants, 
such as Clover, are familiar to most of us. These are 
caused by the stimulation due to colonies of bacteria 
(Bacillus radicicola), which live in the root-tissues as 
internal parasites. These bacteria feed on the sap and 
cell-contents of their host, but they supplement this 
food-supply by absorbing nitrogen direct from the 
atmosphere, which the host cannot do, though it can 
and does use the nitrogenous compounds which the 
bacteria manufacture. It is a case of symbiosis (see 
p. 79), each organism supplying food useful to the 
other; but the significance of the phenomenon is that 
through this agency nitrogen becomes added to the 
soil as the plants decay, and increases its fertility; 
and thus the cultivation of a crop of, say, Lucerne 
becomes a matter of great economic importance in 
