Curvature of Roots . 449 
place as promptly and economically as possible. This is 
accomplished with remarkable rapidity by the process of 
regeneration. Meantime it is also important that while the 
work of repair is going on the root should avoid further 
contact with the source of injury, whether a sharp stone, 
a poisonous substance, or some other agent. This is brought 
about by traumatropic curvature, by means of which the tip 
of the root is turned well away from the injurious influence 
through which it had suffered. The facility with which 
secondary roots are formed might seem to indicate this as 
a more ready method of repairing the injury, but in the case 
of aerial roots these are largely wanting, and in the ordinary 
subterranean root-system it is apparently both more econo- 
mical and more direct to repair the injured tip than to form 
new roots. When, however, for any reason the usual process 
fails, a satisfactory substitute is found in the formation of 
secondary roots. In fact, in certain species, as for example 
Vitis gongylodes , the process of regeneration was less fre- 
quently observed than the growth of secondary roots. 
Thus it appears that in the phenomena of traumatropism 
and regeneration we have merely another chapter in the 
history of the manifold forms of response to external influences 
exhibited by living organs — influences that attract or repel, 
that work swiftly or slowly, that, like the mild warmth of 
spring, gently awaken the normal activities of the plant, or, 
like the fierce cold of winter or a burning caustic, wither and 
destroy, but in all cases influences to which the living proto- 
plasm shows its acute sensibility. Self-defence, the gaining of 
every possible advantage with the least expenditure of energy, 
and the preservation of whatever has been found most useful, 
is here as elsewhere the underlying principle ; and whether 
we regard the long period of developmental history in the 
course of which the higher plants have finally become able, 
in the face of many obstacles, to wrest their nutriment by 
means of special organs from the soil, or in the light of recent 
studies 1 we lay emphasis on their capacity for immediate 
1 See, for example, Sachs, Ueber latente Reizbarkeiten, Flora, 77 Bd., 1893, p. 1. 
