ROOTS AND THEIR USES. 



005 



cells of which form a strongly adhesive surface, by which the plant is 

 firmly fixed. Moreover, when these aerial roots are green they can 

 decompose carbonic acid ; and lastly, being of a spongy texture, they can 

 absorb water with great ease. 



The aerial climbing roots of Ivy have no other function than, by firm 

 adherence to a wall, rocks, or trees, to enable the plant to reach great 

 heights, and so get to the sun and free air. 



Symbiosis. — The nodules on the roots of leguminous plants are now 

 well known to contain microbes, which, by some unknown means, are 

 capable of " fixing " the nitrogen of the air which is then conveyed to the 

 host-plant. This appears to account for the large amount of nitrogenous 

 products in the seeds, &c, of such plants, as of Beans, Peas, and especially 

 Lentils. Hence leguminous species are useful for manuring the soil, 

 especially if dug in green, as they are then very easily decomposed. This 

 was a plan adopted by the ancients, who discovered the value of leguminous 

 plants (as Lupines) without knowing the reason why they are so nutritious 

 when ploughed in. 



Mechanical Force of Roots. — One other feature must be mentioned, 

 and that is the enormous power that roots possess, both under the soil and 

 by insinuating themselves among rocks, under walls, &c. A tree may 

 often be observed raised above the ground, being supported by its great 

 and now exposed, radiating roots. Those roots were at first altogether 

 under the soil, but as they increased in diameter, the earth acting as a 

 fulcrum, they raised the base of the trunk into the air ; the soil being 

 gradually washed away, the present appearance is the result. 



With regard to the destruction of masonry, Sir J. D. Hooker writes as 

 follows, in his "Primer of Botany" (p. 38): "In shrubs and trees the 

 root-fibres as well as the tap-root thicken as they grow, become woody, and 

 displace the earth laterally as well as in front ; and with such force does 

 growth go on that stones of walls are frequently displaced by roots. In 

 tropical countries the destruction of buildings is often caused by the 

 power of growing roots ; and neither conquering nations, nor earthquakes, 

 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 all insidiously 

 begun their work as slender fibres." 



As another remarkable case, the following may be mentioned. In the 

 Pine wood along the northern slope of Table Mountain there is a com- 

 bination of three " Stone " Pines. Their roots are more or less welded 

 together at the base of the trunks. They stand upon a granite platform 

 which the roots have upheaved to an astonishing degree. Many large, flat 

 blocks are standing on edge partly embedded in the trunk ; others are 

 heaved up about them, and the granite base itself is raised throughout an 

 area of about seventy feet in circumference ! 



