MECHANICAL PROBLEMS 103 
large arms of the trees have snapped, because 
the mechanical tissues have proved inadequate 
to meet the unusual demands thus thrown 
upon them. 
When we compare the mechanical arrange- 
ments of the root system of a plant with those 
affecting its aerial portions, we are at once 
confronted by a new set of factors. There are 
two sets of conditions which largely control 
and limit the possible lines of variation in the 
mechanical structure of roots. One of these 
concerns the apical growth of the organ as 
it burrows through the soil, the other relates 
to the pull exerted on the root system by 
the swaying of the parts above ground 
when “they are fretten with the gusts of 
heaven.” 
As regards the growing points of the roots, 
the means for pushing forward in the soil is 
at the same time extremely simple and most 
effective. Unlike the stem, the actually 
elongating portion of the root is situated 
a very short distance behind the conical 
apex, and lies just in front of the zone of 
root-hairs already described (p. 73). The 
latter affords a sort of support which holds 
this part of the root immovable, whilst the 
turgid cells of the very short growing region, 
as they expand in growth, drive the smooth 
conical tip resistlessly forward. Ifthe growing 
region were a long one, as it is in the stem, 
there would be an imminent risk of buckling, 
as may be easily understood if we consider 
