154 ON BIASTREPSIS IN ITS RELATION TO CULTIVATION. 



once apparent. The former are deviations from the type v^rhich 

 show themselves, though not necessarily in a uniform degree, in 

 all the homologous members of the body of an individual, as, for 

 instance, a variation in colour of the flowers or the fruits. The latter 

 are manifested in but few members; thus only a single leaf or flower 

 may exhibit some special peculiarity. I have also found that the 

 partial variations are in a much higher degree dependent upon 

 external conditions than are the individual variations; hence the 

 former are more useful than the latter as a means of studying the 

 effect of external conditions. 



Amongst the partial variations fasciations and twistings of the 

 stem are especially serviceable for this purpose. These phenomena 

 used to be regarded as accidental monstrosities, but I have shown 

 by breeding experiments that they are hereditary^). They are typical 

 examples of partial variation; on each sufficiently branched indi- 

 vidual some of the branches are normal; the normal branches are 

 in fact usually more numerous than the abnormal. 



A few years ago I showed, in Crepis biennis, what results can be 

 obtained by means of selection and cultivation in the case of fas- 

 ciation^); and in an earlier work^) I demonstrated that from a few 



i) Ueber die Erblichkeit der Zwangsdrehungen, 0/era V, S. 159; also, 

 Monstruosites hereditaires offertes en echange aux Jardins Botaniques, 

 Opera VI, p. i. — [Note. The expression 'twisting of the stem', used 

 above in the text, is intended to render into English the term 'Zwangs- 

 drehung', applied to this abnormality by Al. Braun (Monatsber. d. k. 

 Akad. in Berlin, 1854), for which there is no recognized English equi- 

 valent (see Masters, Vegetable Teratology); it might perhaps be well to 

 adopt the term 'biastrepsis', proposed by Schimper (Flora, 1854, p. 75). 

 The addition of a brief description of this 'Zwangsdrehung' or 'biastrep- 

 sis', in its characteristic form, may be of use to the reader. It occurs 

 only in plants, the shoots of which have opposite or whorled leaves ; the 

 phyllotaxis becomes spiral instead of verticillate, the successive leaves of 

 the spiral being connected by their bases. The effect of this cohesion 

 of the leaf-bases is to prevent the normal elongation of the internodes, 

 which therefore become spirally twisted and often much dilated and 

 otherwise monstrous in form. When, as is sometimes the case (e. g. 

 Dipsaats), the stem is hollow, the pith-cavity is normally interrupted by 

 diaphragms at the successive nodes; but in the twisted stems the pith- 

 cavity is continuous, the diaphragms of the normal stem being represented 

 by a rib projecting into the pith-cavity and following the course of the 

 leaf-spiral. — Ed.] 



2) Sur les courbes Galtoniennes des monstruosites, Oj>era V, p. 570. 



3) Monographie der Zwangsdrehungen, Oj>era V, p. 22^2. See also, 

 'Eine Methode Zwangsdrehungen aufzusuchen', p. 478, and 'Bijdragen tot 

 de leer van de Klemdraai', p. 407. 



