ig6 



MORPHOLOGY OF MEMBERS. 



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° would be sufficient to suiter the divergence from f to |, a similar 

 displacement of 1-3° would change the divergence from ^^ to ^j. When the phyl- 

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

 extremely smaH 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 in- 

 terest so far as it makes it seem doubtful 

 whether certain complicated phyllotaxes 

 are always due to the original arrange- 

 ment of the members \ 



(4) It must be observed whether the 

 position of newly-formed members or the 

 subsequent change shows any relation to 

 the direction of the force of gravitation, of 

 the light which falls upon them, or of any 

 pressure acting from without^. The effect 

 of the force of gravitation is that primary 

 shoots which are in the main upright put 

 forth leaves spreading 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 ar- 

 rangement of leaves on the latter in two 

 rows, or one which is divided into two 

 equal halves by a plane cutting the stem 

 longitudinally, as Salvmia, Marsilea, Poly- 

 podium aureum, Pteris aquih'na, &c. When 

 vertical primary shoots with leaves in 

 several rows bear secondary horizontal 

 branches with leaves in two rows, this 

 relationship is less clear, as in the cherry- 

 laurel, sweet chestnut, hazel, &c., because 

 an influence independent of gravitation 

 must in these cases be presumed to be 

 exercised by the primary upon the lateral 

 axis, as is shown by the position of the 

 leaves in the lateral buds before unfolding (see Fig. 155, p. 208). 



(5) It must further be observed whether the first appearance of lateral 



Fig. 154 —Diagframmatic representation of the orthostichies 

 bf a i phyllotaxis in Pandantis utilis ; A before, B after the 

 torsion of the stem. Each of the orthostichies /, //, /// is indi- 

 cated by a double line ; the genetic spiral is simple ; where 

 it crosses the orthostichy, the leaf-insertions are indicated by 

 figures. 



' [See Airy, Proc. Royal :Soc. /. c] 



' Hofmeister (Allgemeine Morphologic, §§23, 24) has collected a series of facts which show 

 relationships of this kind ; but, both with reference to the individual facts and to the interpretation 

 which he gives, I am decidedly of a different opinion, the reasons for which would carry me loo far. 

 (Fit/ffi«/ra, Sect. 27.) 



