134 INFLUENCE OF THE EXTERNAL CONDITIONS ON GROWTH 



effects must be distinguished, as far as possible, from the true wound-reaction, 

 although a clear distinction cannot always be made. 



The wound-reaction is an important means of self-regulation, for it is 

 not only produced by external injuries, but also to a certain extent by the 

 natural death or removal of particular cells or organs. In all cases the 

 reaction is the result of disturbances modifying certain factors and producing 

 alterations in the vital activity. Thus transpiration and diosmosis are 

 affected by the exposed position of an externally injured region, and it is 

 probably these factors which determine the formation of cuticle on exposed 

 cells. The changes of the tissue-strains produced when an organ is removed 

 also act as stimuli, and the removal of an organ may permit growth where 

 mechanical pressure rendered it impossible previously. Thus the pith grows 

 somewhat when isolated from young stems which have themselves ceased 

 to grow. Similarly the cambium, which is only capable of producing an 

 increase of thickness in adult stems, grows freely on a cut surface and 

 spreads out to form a callus. Special stimuli are, however, often involved, 

 for injuries may induce growth in single cells or tissues, which were quiescent 

 in the adult plant, although no mechanical resistance was offered to their 

 growth. This applies to the formation of tyloses, which only appear when 

 the wood has attained a certain age, or when an injury acts as a stimulus 

 to their formation 1 , although the open lumina of the tracheae have been 

 available for a long time previously. 



Similarly the formation of cork over exposed surfaces is not the direct 

 and unavoidable result of the exposure, for the leaf-scars of some leaves do 

 not become covered by cork, nor do the cells surrounding the intercellular 

 spaces. In some plants, moreover, an internal formation of cork occurs, and 

 a cork-layer is formed around an inserted wooden peg, but not around the 

 penetrating root of a parasite. It is, however, always possible that in certain 

 cases the increased transpiration may induce or accelerate the formation of 

 cork 2 . 



Traumatic reactions may not only lead to changes of shape, but also may 

 produce either an acceleration or a retardation of growth. The stretching growth 

 of a decapitated root is only slightly retarded 3 , whereas when the tip is cut off 

 the first seedling leaf of Avena sativa, a pronounced but temporary slowing of 



1 Cf. Frank, Krankheiten d. Pflanzen, 2. Aufl., p. 35 ; Maule, Bibl. hot., 1895, Heft 33 ; 

 Massart, La cicatrisation chez les vegetaux, 1898, p. 43 ; Warburg, Ber. d. Bot. Ges., 1893, 

 p. 427; Bd. II, p. 51. Cf. Kny, Die Verwachsungen an d. Wurzelhaaren d. Marchantiaceen (repr. 

 from Sitzungsb. d. Bot. Vereins d. Prov. Brandenburg, 1879, Bd. xxxi) ; Dixon, Notes from the 

 Botanical School of Trinity College, Dublin, 1901, p. 141 ; Nordhausen, Jahrb. f. wiss. Bot, 1900, 

 Bd. xxxv, p. 372. Cells penetrate others in many algae, in the growth of the zoosporangium 

 of Safrolegnia, and in the formation of new rhizoids from the inner tissue-cells of Marchantia. 



a Kny, Ber. d. Bot. Ges., 1889, p. 154. 



s Czapek, Jahrb. f. wiss. Bot., 1895, Bd. xxvn, p. 246. 



