CORRELATION 165 



stimulus acts directly or indirectly upon the organ affected. Whenever 

 a depression of the activity of one organ affects others, we may with some 

 certainty conclude that the influence is exercised indirectly. 



Considering the immediate and far-removed actions without regard to 

 the mode in which they are transmitted, but only from the point of view 

 of purpose, it might be expected that remote reactions should be more 

 directed towards the maintenance of a harmonious co-operation between 

 different organs, while reactions in neighbouring parts are of more 

 importance in directing the character and amount of growth. For example, 

 it is the neighbouring tissues which influence the mode of differentiation of 

 the meristems at the apex of root and shoot in Phanerogams and at the 

 growing-point of the thallus of Marchantia. It is, however, often the case 

 that a remote correlative effect involves a considerable modification of the 

 shape of the organ affected, as for example when the removal of the 

 primary and secondary axes induces fasciation in Barkhausia taraxifolia T . 



Various instances of the complex and often far-reaching reactions and 

 interactions involved in correlation may be given 2 . Thus the stimulus of 

 fertilization causes the development of the embryo, which again causes the 

 carpel to ripen into a fruit, and this may so influence translocation and the 

 processes related to it as to cause changes of metabolism, storage, and 

 growth in far-removed organs such as bulbs and tubers. Similar complex 

 relationships are concerned when a stem is removed whose presence caused 

 certain buds to lie dormant, for influences radiate from the awakened bud, 

 just as they do from the fertilized ovum, tending to produce such changes 

 as are required by the new conditions. As in the above instance, no 

 stimulatory effect can be restricted to the point of application in plants 

 whose protoplasts are in vital continuity, although perceptible results may 

 only appear at the point immediately affected. 



Irritability is an essential property of the protoplast, whose vital activity 

 in fact results from the harmonious and self -regulated interaction of its 

 component parts and organs. Observations on plants as well as on the 

 lower animals have shown that irritability and conductivity, and hence also 

 correlation, are not necessarily bound up with the existence of nerves, 

 nerve-centres, and sense-organs. A particular organ, as for example the 

 fertilized ovum, may, however, act as a stimulatory centre, and in general 

 ic needs of the organism act as stimuli exciting reactions directed 

 towards their satisfaction. The fact that it is the activity, and not the mere 

 :istence of an organ, which is of importance in regulation, is admirably 

 shown by the fact that the same results are produced when the growth 



Ge"neau de Lamarliere, Compt. rend., 1899, T. cxxvm, p. 160. On fasciation cf. de Vries, 

 t. Centralbl., 1899, Bd. LXXVII, p. 289, and the literature there given, 



2 Pfeffer, Die Reizbarkeit d. Pflanze, 1893, p. 93 (repr. from Verb. d. Ges. deutsch. Naturf. 

 u. Aerzte, 1893). 



