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, e.g., 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 

 Vbchting^. 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 gro^vn stems cannot be destroyed ; it is the ex- 

 pression of previous development. Vochting 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 



^ KlebR, WillkUrliche Entwickelung, -p. 100; also, " Probleme der Entwickelung," £ioZ. 

 Centralhl. 1904, p. 610. 



' See the classic work of Viichting, Veher Organbildung im Pflanzenreich, i. Bonn, 

 1R88 ; also Bot. Zeit. 1906, p. 101 ; cf. Goebel, Experimentelle Morphologie, Leipzig and 

 Berlin, 1908, Section v, Polaritiit. 



