CORRELATIONS 331 



branches and of laterally-placed flowers is also due to the correlative influences 

 of the main axis, and in this relation it should be remembered that normally 

 dorsiventral flowers, such as those of Orchidaceae and Scrophulariaceae, generally 

 become radial if they once become terminal (peloria). 



We have now become acquainted with numerous correlations among pheno- 

 mena of regeneration appearing after the infliction of wounds, but these regenera- 

 tions are not the first or only consequences of injury. If a leaf -blade be cut off 

 serious disturbances must ensue in the petiole and the portion of the stem im- 

 mediately associated with it, which express themselves in reactions easily re- 

 cognized. Such reactions, consisting in the throwing off of the petiole in the case 

 of the leaf which has lost its lamina or the stem which has lost its leaves, have 

 already been referred to. If, however, all the leaves are not removed from the 

 stem there is no reason why the shoot should die off ; on the other hand, it no 

 longer develops conducting tissue to the removed leaves. There is also a relation 

 between the leaf and the leaf -base on the stem, which may be studied very readily 

 in the epicotyl of Phaseoliis mtiUiftonts. If we remove one of the two young 

 primary leaves at the first node of the epicotyl and at the same time carefully 

 cut off the stem apex (whose further development would complicate the experi- 

 ment) we see a marked reduction in the diameter of the vascular elements on 

 one side of the epicotyl, from the base to the apex, while those on the side on 

 which the leaf still remains are developed normally and also show indications 

 of secondary growth (Jost, 1891). 



The conclusion to be drawn from this experiment is undoubtedly that the 

 result is not due to the injury but to a suspension of function, and especially of 

 the growth in the leaf. This is evidenced by the fact that the same stem-structure 

 may be induced if the leaf be not cut off but only prevented from further growth 

 by enclosure in plaster of Paris. In other cases also the removal of an organ is 

 followed by purely mechanical growth retardation. Thus Hering (1896) has 

 shown that the cotyledon of Streptocarpus, which normally does not develop, 

 may be made to do so either by cutting off the one which does, or by enclosing 

 it in plaster of Paris ; Winkler also showed that leaf-regeneration could be 

 induced in Cyclamen when he enclosed the leaves in plaster of Paris with- 

 out injuring them. Since in this case we are dealing with fully-developed 

 organs the plaster could not have acted as in Streptocarpus or Phaseolus by 

 retarding growth, but rather by inducing a cessation of the junction of the 

 organ. He obtained similar results by using instead shells of collodium and 

 shellac. We thus arrive at the conviction, which, however, still demands ex- 

 perimental confirmation in many points, that regenerations may be induced 

 not only by removal of an organ but also by rendering it inactive. [As to the 

 various factors concerned in regeneration compare Klebs (1903), Goebel (1905), 

 and MacCallum (1905).] In the case of the plant it is difficult to distinguish 

 whether an organ is removed or merely has its functional activity retarded. 



Organs which are not functional are usually abstricted, as we have already 

 seen in the case of petioles and stem-bases. Intact foliage-leaves in sensitive 

 plants, e.g. Mimosa, are rapidly thrown off if they be prevented from assimilating 

 by withdrawal of carbon-dioxide or by being kept in darkness (Vochting, 1891 ; 

 Jost, 1895). Certainly all plants are not so sensitive ; evergreen leaves, for 

 instance, which live the whole winter through without manifesting any activity, 

 may be kept in the dark for months without dropping off. 



We arrive at an exactly opposite result to that which we obtain with leaves 

 prevented from assimilating, if organs which have lost their original function 

 and are, in consequence, dying, take on new functions. Thus Vochting (1887) was 

 able by employing special artificial means to cause tubers of a potato which, owing 

 to germination, had dried up and were dying off to remain alive for a year more, 

 and he obtained even better results with Oxalis crassicaulis (1899), a plant 



