248 PROCESS OF DEVELOPEMENT. CHAP. IV. 



always the more vigorous the nearer they are to the 

 trunk, but the lateral branches of horizontal roots 

 are the less vigorous the nearer they are to the 

 trunk. In the former case the increased luxuriance 

 is perhaps owing to the easy access of oxygene in 

 the upper divisions; but in the latter case the in- 

 creased luxuriance of the more distant divisions is 

 not so easily accounted for, if it is not to be at- 

 tributed to the more ample supply of nutriment 

 which the fibres meet with as they recede from the 

 trunk, particularly if you suppose a number of them 

 lying horizontally and diverging like the radii of 

 a circle. 



Accidental But the direction of roots is so liable to be affected 

 ' by accidental causes, that there is often but little 

 uniformity even in roots of the same species. If 

 plants were to be sown in a soil of the same density 

 throughout, perhaps there might be at least as much 

 uniformity in the figure and direction of their roots, 

 as of their branches ; but this will seldom happen. 

 For if the root is injured by the attacks of insects, 

 or interrupted by stones, or earth of too dense a 

 quality, it then sends out lateral branches, as in the 

 above cases ; sometimes extending also in length by 

 following the direction of the obstacle, and some- 

 times ceasing to elongate, and forming a knot at 

 the extremity. But where the soil has been loosened 

 by digging or otherwise, the root generally ex- 

 tends itself to an unusual length. This Du Hamel 

 has illustrated by the following cases : If a trench 



