CURIOUS MODES OP GROWTH OP ROOTS. 83 



approached the surface of the water. It is not improbable that 

 the constant ascent of vapour into the air in that direction, may 

 be in part the cause of this curious mode of growth. An in- 

 stance occurs in Leigh "Woods near Bristol, which remarkably 

 illustrates this tendency of roots, to grow towards the spot most 

 fitted to afford them nutriment. In a little hollow on the top of 

 the shell of an old Oak (the outer layers of which, however, and 

 the branches, are still vegetating) the seed of a Wild Service-tree 

 was accidentally sown. It grew there for some time, supported, 

 as it would appear, in the mould formed by the decay of the 

 trunk on which it had sprouted ; but this being insufficient, it 

 has sent down a large bundle of roots to the ground, within the 

 shell of the Oak. These roots have now increased so much in 

 size, that, as they do not subdivide until they nearly reach the 

 ground, they look like so many small trunks. In the soil, how- 

 ever, towards which they directed themselves, there was a large 

 stone, about a foot square; and, had their direction remained 

 unchanged, they would have grown down upon this. But about 

 half a yard above the ground, they divide, part going to one side, 

 and part to the other ; and one of them branches into a fork, of 

 which one leg accompanies one bundle, and one the other ; so 

 that, on reaching the ground, they enclose the stone between 

 them, and penetrate on the two sides of it. 



110. This example serves to show another fact, that it is 

 not, in every case, that we are to regard the parts of the axis 

 which are above ground, as Stems, and those which are beneath 

 it, as Roots. There is a tree peculiar to tropical climates, called 

 the Pandanus or Screw Pine, in which the roots are always 

 formed in somewhat of this manner. The stem is smallest at its 

 lowest part, and it enlarges considerably above ; hence it would 

 be very unsteady, without some additional support ; and this is 

 provided for, by the transmission of the roots, not only from the 

 bottom of the stem, but at different parts of its ascent. These 

 grow downwards in the air, and are provided at their extre- 

 mities with a kind of cup, which catches the rain and dew by 

 which they are partly assisted in their elongation ; when, how- 

 ever, they have reached the ground, this falls off, and their 



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