CORRELATION AND REPRODUCTION 



163 



of the carpel, and if the growing zone of a seedling-stem has its growth 

 mechanically prevented, the growth of the free apical portion is also 

 considerably retarded 1 (Fig. 29). These correlative stimuli require 

 some time for their action to be completed, and in some cases a transitory 

 retardation may be followed by an acceleration of growth. A slight 

 reduction in the size of the root-system may naturally produce no 

 perceptible effect upon that of the shoot, while the 

 effect of a transitory retardation may be readjusted 

 by a subsequently increased activity of growth 2 . 



Self-regulation is possible only when the functions 

 concerned are not engaged to the maximal possible 

 extent, and when the disturbing influence can be 

 equilibrated by the acceleration of certain functions 

 and the retardation of others. Hence it might be 

 expected that growth should be accelerated when a 

 defect was to be repaired, and that a restricted 

 growth in one part should induce a retardation in 

 the growth of related parts. The same end may 

 be attained in various ways, such as by the excite- 

 ment, acceleration, or modification of growth in an 

 existent organ, or by the induction of new growths. 

 To the latter class belong the healing of wounds, 

 and the re-formation of the cell- wall around naked 

 protoplasts. 



All reactions in the last resort involve either a 

 change in the activity of growth and metabolism or 

 in their character, so that it is possible to distinguish 

 between quantitative and qualitative correlative reac- 

 tions, although no sharp distinction exists between 

 them 3 . Under certain circumstances one organ may 

 vicariously assume the function of another, as when 

 the suppression of the root-tubers causes a portion s <- 

 of the stem to swell up and become filled with food- 

 materials 4 . It is, however, not uncommonly the case 

 that morphologically similar organs are specially 

 differentiated for different purposes, and that dissimilar organs acquire a 

 close analogy owing to their similarity of function. 



1 Hering, Jahrb. f. wiss. Bot, 1896, Bd. XXIX, p. 157. [The decreased growth may in part be 

 due to a diminution of the water-supply.] 2 Cf. the literature quoted on p. 158. 



3 Goebel, Organography, 1900, I, p. 207. Goebel applies the term 'compensations' to 

 quantitative reactions, but since we are really dealing with internal phenomena, it is difficult 

 to see any sound basis for Herbst's distinctions between compensations and alteration?, terms which 

 apply only to the visible results. (Cf. Herbst, Biol. Centralbl., 1895, Bd. xv, p. 724.) 



* Vochting, Jahrb. f. wiss. Bot., 1899, Bd. xxxiv, p. 77. 



M 2 



