58 PLANT LIFE ON THE FARM. 
high or too low a temperature. During their passage 
through the soil, the roots must be constantly subjected 
to variations of temperature, first on one side and then 
on another, these variations giving rise to some of the 
curvatures and bends of the rootlets. The effect of an 
excessive amount of heat in the soil upon the germination 
of seedlings has been studied by M. Prillieux, and is of 
interest as indicating the conditions under which tuberous 
roots and root stocks may, under certain circumstances, 
be formed. When seedlings of French beans and vege- 
table marrows were grown in an overheated soil, the 
caulicle or portion of the stem above the root and between 
it and the seed-leaves became preternaturally swollen 
and tuberous, while growth in hight was arrested. The 
increased development arising from the heated soil took 
place, therefore, in the very same organs which constitute 
the so-called ‘‘bulbs” of turnips or ‘‘roots” of swedes 
or mangels. The increased volume is due principally to 
an excessive development of existing cells rather than to 
a multiplication of new ones. 
The Action of Moisture on Roots.—Much more 
obvious to the general observer is the action of moisture 
on roots. The distance to which roots will travel in 
search as it wereof water, and the way in which luxuri- 
ant growth and intricate ramification are promoted, when 
access to it is obtained, are familiar facts. Too frequently 
drain pipes get choked with a mass of roots whose 
structure has been changed, and whose excessive growth 
has been stimulated by the presence of copious supplies 
of moisture. If there is an equal supply of water all 
round, the growth of the roots will be uniform ; but if, 
as is more often the case, there is more water on one side 
than on the other, then the root will curve to the side 
where there is the fullest supply, and the power thus 
exerted to get at the water is greater than that of gravity. 
