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. If the 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 



