MECHANICAL STRUCTURES IN THE VEGETATIVE ORGANS. 383 



horizontal roots just below the surface, while pile-roots descend 

 vertically from them underground. They are often exposed when 

 a cutting is made for a road, as in Bournemouth. 



Roots of dicotyledonous trees, of course, supply the main sources 

 of resistance to the wind. They usually run more or less superficially 

 and horizontally in open ground ; so that, whichever way the wind 

 blows, the roots on the same side afford a powerful " pull " against it. 

 But, if a tree grows on the edge of a steep bank, one or more roots 

 run vertically downwards on the exposed side, while others lie more 

 or less obhquely, penetrating the adjoining field. The former would 

 be pile-roots, resembHng the piles supporting a pier, one end of which 

 is on the land. 



Upheavals by Leverage. — What maybe called ''imitative" pile- 

 roots may often be seen, but these are due to the exposure of originally 

 subterranean roots, by their being elevated by the leverage through 

 the power of growth and the resistance of the underlying stratum. 



Sir J. D. Hooker, in his " Primer of Botany," observes : " In 

 tropical countries the destruction of buildings is often caused by the 

 power of growing roots ; and neither conquering nations, nor earth- 

 quakes, nor fires, nor tempests, nor rain, nor all put together, have 

 destroyed so many works of man as have the roots of plants, which 

 have insidiously begun their work as slender fibres." * 



All such exhibitions of force are the result of leverage, in which the 

 energy of the roots growing downwards is more or less arrested by the 

 obstruction of the soil or rock. It is then diverted into a lifting 

 power, the resistance of the rock being the fulcrum. Trees may 

 often be seen with the upper parts of their roots exposed. This is 

 the result of their fitting power. It is a common thing to see a wall, 

 near which a tree is growing, cracked from top to bottom by the same 

 cause. 



The Mechanism of Roots. — As a strain upon a root partakes of 

 the nature of a " pull " rather than lateral pressure, this is met by 

 the whole bundle of mechanical tissue being developed in the middle, 

 the pith being almost or entirely absent. Besides wood-fibres, scleren- 

 chymatous tissues may be developed besides a strongly thickened 

 endoderm. 



Cylindrical Stems devoid of Pith. — Many cylindrical and hollow 

 stems are comparatively slender and weak, as straw and bamboo stems. 

 These have to resist the lateral pressure of the wind. The first 

 additional structures are the diaphragms placed at intervals at the 

 " nodes," composed of strong woody strands crossing from one side to the 

 other. This method is not usually imitated by engineers ; for to unite 

 sections of iron tubing a flat but ring-Hke piece of iron is placed 

 between the two ends, these being provided with flanges, which are 

 then riveted to the ring between the latter, the whole being 

 surrounded by plates, called a " collar." This may be compared to 



* Primer 0^' Botany, p. 38. 



