THE ROOTS OF VASCULAR PLANTS. 221 



The general direction of roots is, as we said at the 

 commencement, to descend towards the centre of the 

 earth ; but if we compare them with one another, we 

 see that they present differences in this respect ; some, 

 and these are in general those of the two great classes 

 which are the least branched, have a tendency to descend 

 almost vertically, and they depart very little from this 

 direction. Branching roots, on the contrary, present in 

 general a Tap-root (pivot) that is to say, the principal 

 trunk of the root, which has a tendency to descend ver- 

 tically ; but the lateral branches are always obliged to 

 separate more or less. 



When they proceed from the principal trunk at a very 

 acute angle, and tend successively to be directed towards 

 the centre of the earth, they come under the general 

 idea of roots ; but it sometimes happens that they sepa- 

 rate from the trunk at a right angle, or nearly so, and 

 are prolonged, at least the upper ones, nearly parallel 

 with the surface of the ground ; we see this in the Ro- 

 binia, the Elm, &c. ; roots of this kind are said to be 

 horizontal, spreading, or creeping. As they are found 

 near the surface, they are often exposed, either by natu- 

 ral accidents or by the hand of man, and then they 

 readily shoot out new stems. In this case the new indi- 

 vidual, thus developed, may, spontaneously or artificially, 

 be divided from that which gave origin to it, and grow 

 separately. In roots of this kind the tap-root frequently 

 elongates but little, and sometimes even dries up or be- 

 comes hard at its extremity. 



