MECHANICAL PROPERTIES OF ROOTS 287 



of the organ ; vessels may completely occupy the centre, 

 or there may be a certain amount of pith. The secondary 

 thickening, if it takes place, increases the thickness of this 

 central core ; sclerenchyma may be added in the cortex. 



Fixation to the soil particles takes place by the root 

 hairs, but this may not have much actual importance in 

 securing the plant in the soil. The innumerable branching 

 roots and rootlets, following a sinuous course in every 

 direction, offer an enormous frictional resistance to with- 

 drawal. It is, of course, to the laterals that stability is 

 chiefly due ; even a very strong vertical tap would be 

 unsuited to maintain a tree erect. The multitude of laterals 

 also ensures a distribution of the strain. In herbaceous 

 plants and shrubs the root system is perfectly efficient ; 

 only with the immense weight and resistance to wind 

 offered by the foliage of a tree does the root system fail to 

 meet all possible demands, and rupture may occur in a 

 storm. A wind-felled spruce, however, lifts the soil with 

 it ; the surface distribution of the laterals, and not their 

 strength, is here faulty. This is the case only in shallow 

 root systems. 



A special mode of securing stability is shown by some 

 plants which form " buttress " or " prop " roots. In the 

 maize roots grow out from the lower nodes above ground 

 and arch away from the stem before entering the soil. The 

 mangroves offer varied examples. In Brugmera gymnorhiza, 

 which also possesses knee-shaped pneumatophores, prop 

 roots, like thick blades, slant away from the base of the tree 

 into the mud. In Rhizophora mucronata, according to 

 Schimper, *' a regular scaffolding of bow-shaped stilt roots 

 supporting the stem represents a complete system of anchors, 

 which is strengthened by new roots growing down from the 

 branches to support the growth of the crown." The man- 

 groves require such special supports to withstand the wash 

 of tide and wave. Among our own trees the elm sometimes 

 shows quite characteristic buttress roots formed by excessive 

 growth in thickness on the upper edge. 



Contractile Roots. — Mention may here be made of 



