82 DIRECTION OP GROWTH OP ROOTS. 



that, in which the earth is loosened by the flow of water ; 

 towards the source of that moisture, therefore, the grovvtli of the 

 roots will be directed. 



108. The same principle has another curious application. 

 Roots have been known to insinuate themselves into the crevices 

 of walls, or even into chinks in the stones themselves ; and to 

 burst asunder the walls of these after some time. Now when a 

 root meets with such an obstacle as this in its growth, it is turned 

 aside for a time, and endeavours, as it were, to creep round it. 

 But should a chink give the opportunity for the new tissue to be 

 deposited without obstacle, in its original direction, the root will 

 find its way into it. The fibril will grow, by the nourishment 

 sent to it, and by its own absorption, until it can no longer in- 

 crease, without the separation of the walls which confine it ; and 

 this is accomplished by the force with which it imbibes fluid, 

 which causes it to distend itself with great violence.* This dis- 

 tention is of the same character as that of a piece of dry wood, 

 when exposed to moisture. 



109. The foregoing explanations will apply to a case of no 

 unfrequent occurrence, in which a tree growing on one side of a 

 road sends out roots to a ditch or stream on the other, these 

 roots dipping deep beneath the hard bed of the road, and rising 

 again on the farther side. It is evidently by the slight drainage 

 or percolation of water, which will take place along this line, 

 that the roots follow the same course. A more remarkable case, 

 however, which has been more than once observed, is where the 

 roots direct themselves along a naked rock, to reach water at a 

 distance of perhaps twenty feet : or where, as in a case recently 

 seen by the Author, a tree growing near the side of a well, has 

 sent down a root through a narrow chink in its side ; which root, 

 after descending for several feet without subdivision, has thrown 

 out a vast number of small branches and fibres, as soon as it 



* It is by this same kind of force, resulting from Capillary Attraction, (see 

 MECHAN. PHILOS. . 30) that mill-stones are split in quarries. A long cylinder 

 is first cut out; and grooves are then chiselled in its circumference, at the points 

 where it is desired to divide it. Wedges of wood are then driven into the grooves, 

 and these are moistened ; by their violent expansion, the stone is split into the 

 required number of parts. 



