234 Influence of Environment on Plants 
The widely spread power of reacting to wounding affords a very 
valuable means of inducing a fresh development of buds and roots 
on places where they do not occur in normal circumstances. Injury 
creates special conditions, but little is known as yet in regard to 
alterations directly produced in this way. Where the injury con- 
sists in the separation of an organ from its normal connections, the 
factors concerned are more comprehensible. A detached leaf, eg., is 
at once cut off from a supply of water and salts, and is deprived of 
the means of getting rid of organic substances which it produces; 
the result is a considerable alteration in the degree of concentration. 
No experimental investigation on these lines has yet been made. 
Our ignorance has often led to the view that we are dealing with 
a force whose specific quality is the restitution of the parts lost by 
operation; the proof, therefore, that in certain cases a similar pro- 
duction of new roots or buds may be induced without previous 
injury and simply by a change in external conditions assumes an 
importance}. 
A specially striking phenomenon of regeneration, exhibited also 
by uninjured plants, is afforded by polarity, which was discovered by 
Viéchting®. It is found, for example, that roots are formed from the 
base of a detached piece of stem and shoots from the apex. Within 
the limits of this essay it is impossible to go into this difficult question ; 
it is, however, important from the point of view of our general survey 
to emphasise the fact that the physiological distinctions between base 
and apex of pieces of stem are only of a quantitative kind, that is, 
they consist in the inhibition of certain phenomena or in favouring 
them. As a matter. of fact roots may be produced from the apices 
of willows and cuttings of other plants; the distinction. is thus 
obliterated under the influence of environment. The fixed polarity 
of cuttings from full grown stems cannot be destroyed; it is the ex- 
pression of previous development. Véchting speaks of polarity as a 
fixed inherited character. This is an unconvincing conclusion, as 
nothing can be deduced from our present knowledge as to the causes 
which led up to polarity. We know that the fertilised egg, like the 
embryo, is fixed at one end by which it hangs freely in the embryo- 
sac and afterwards in the endosperm. From the first, therefore, 
the two ends have different natures, and these are revealed in the _ 
differentiation into root-apex and stem-apex. A definite direction 
in the flow of food-substances is correlated with this arrangement, 
and this eventually leads to a polarity in the tissues. This view 
1 Klebs, Willktirliche Entwickelung, p. 100; also, ‘‘ Probleme der Entwickelung,” Biol. 
Centralbl, 1904, p. 610. 
2 See the classic work of Véchting, Ueber Organbildwng im Pflanzenreich, 1. Bonn, 
1888 ; also Bot. Zeit. 1906, p. 101; cf. Goebel, Experimentelle Morphologie, Leipzig and 
Berlin, 1908, Section v, Polaritit, 
