RELATIVE POSITIONS OF LATERAL MEMBERS. 1 75 



Structure on one side than on the other ; and still more commonly by torsion 

 around its own axis of growth. By such torsions lateral members, arranged at first 

 exactly in straight rows, become displaced so that the orthostichies appear as if 

 wound spirally round the axis. This occurs, for instance, according to Nageli 

 and Leitgeb, in the root-systems of Ferns, Equisetaceae, and Rhizocarps, as well as 

 in the three-rowed phyllotaxis of the Moss Fontinalis antipyretic a, according to 

 Leitgeb. But the most striking example is furnished by the stem of the screw-pine 

 Pandanus utilis ; in the bud, the numerous and already strongly developed leaves 

 stand, as is shown by the transverse section, in three perfectly straight lines with 

 the phyllotaxis ^ ; but, as the development of the stem advances, it undergoes so 

 severe a torsion that the three orthostichies are transformed into three strongly 

 curved spiral lines running round the stem. In these and similar cases the change 

 in the relative positions caused by the torsion of the axial structure is easily and 

 certainly established. But when the positions are so related to the apex of the 

 axial structure that the angle of divergence cannot be accurately estimated by 

 an apical view from above, it must remain uncertain whether the position of the 

 mature members is unchanged or has been altered by lateral displacement and 

 torsion of the axis. A displacement, for instance, of about 9" of the circumference 

 of the axis would be sufficient to alter the divergence from f to ^, a similar dis- 

 placement of 1-3"' would change the divergence from y% to ^\. When the phyl- 

 lotaxis is very complicated and the number of the longitudinal rows very large, 

 extremely small and almost inappreciable distortions are sufficient to destroy the 

 original arrangement and to bring into existence altogether different systems of 

 parastichies. This observation is of interest so far as it makes it seem doubtful 

 whether certain complicated phyllotaxes are always due to the original arrangement 

 of the members \ 



(4) It must be observed whether the position of newly-formed members or 

 their later transformation shows any relation to the direction of the force of gravity, 

 of the light which falls upon them, or of any pressure acting from without^. 

 The effect of the force of gravity is that primary shoots which are in the main 

 upright put forth radiating leaves on all sides ; while such as have a decidedly 

 horizontal growth, in which a rooting under-side is contrasted with an upper-side, 

 usually show an arrangement of leaves on the latter in two rows, or one which is 

 divided into two equal halves by a plane cutting the stem longitudinally (<?. g. 

 Salvinia, ]\Iarsilea, Poly podium aureum^ Pteris aquilina, &c.). The horizontal lateral 

 axes with leaves arranged in two rows on vertical primary shoots which have the. 

 leaves arranged in several rows (as in Prunus Lauro-cerasus, Castanea vesca, Corylus,: 

 &c.) show this kind of relationship less clearly^ because an influence independent 

 of gravity must in these cases be presumed of the primary upon the lateral axis, as. 

 is shown by the position of the leaves in the lateral buds before unfolding (see: 

 Fig. 147, p. 187). 



^ [See Airy, Proc. Royal Soc. / c] 



^ Hofmeister (AUgemeine Morphologic, §§ 23, 24) has collected a series of facts which show- 

 relationships of this kiad ; but, both with reference to single facts and to the interpretation which he 

 gives, I am decidedly of a different opinion, the reasons for which would carry me too far. (Cf. infra, 

 sect. 27.) 



