1876.] 



Leaf -arrangement of the Crowberry. 



159 



In all these instances the striking peculiarity to be observed is that 

 the arrangement passes from an order belonging to one phjUotactic 

 series {e.g. from the order |- in the primary series |, ^, -|, &c.) to an 

 order belonging to another phyllotactic series {e. g. to the order |- in the 

 secondary series ^, 5, |, &c.), and that this is a phenomenon which could 

 not result from uniform vertical condensation of the lower arrangement; 

 whereas in other plants the ordinary transition is from one order to 

 another of the same series {e.g. from f to |, y^^-, &c.), and is such as 

 luoulcl result from uniform vertical condensation of the lower arrange- 

 ment (as the author has shown in a pp.per read before the Eoyal Society 

 on the 30th April, 1874 : see Proc. vol. xxii. p. 298). 



In order to examine the mode in which the above transitions were 

 effected, the author constructed an instrument (described below) by 

 means of which he obtained diagrams, pricked on paper, of the leaf- 

 arrangement of the fifty specimens. On comparison of these diagrams it 

 was found that, in every case where the details could be traced, a gTadual 

 dislocation (so to speak) took place between two adjacent spirals belong- 

 ing to one of the two sets of spirals \\ith least angular divergence, whereby 

 the other set became deranged and gave rise to new sets having different 

 numbers. Thus the transition from the arrangement f (in which the 

 spirals of least angular divergence are 2 in one direction and 3 in the 

 other) to the arrangement |- (in which those spii'als are 3 in one direction 

 and 4 in the other) was effected by an apparent slip between two of the 

 3 spirals, whereby the 2 spirals became deranged and helped to form a 

 new set of 4 spirals, which with the old persistent set of 3 formed the 

 spirals of least angular divergence in the new arrangement I-, w^hile a 

 new primary spiral arose, turning in an opposite direction to the old. 

 (A diagram is required to make thi^ clear.) In all these transitions the 

 change of order is brought about not by any agency affecting all parts of 

 the system uniformly all round the axis, but by a disturbance of the 

 relative position of leaves along one special spiral tract. 



The variations described by the Eev. Gr. Henslow (Trans. Linn. Soc. 

 vol. xxvi. p. 647) as occurring in the leaf -arrangement of Helianthus 

 tuherosus appear to be similar in character to those of Emjjetrum nigrum 

 but much more limited in range. The author has met with a few other 

 instances of the same kind in other plants. 



It is noteworthy, in the above transitions, that the primary spiral of 

 the basal order is lost at the first change and gives place to a new one 

 turning in the opposite direction, which, again, is replaced by another 

 at the next change : in fact, in the crowberry, no rank or set of ranks is 

 found to possess enduring value, but all are liable to derangement. They 

 appear to be only the local result of the geometrical conditions of mutual 

 accommodation of contiguous leaves under mutual pressure {luJiich brings with 

 it the common need of economy of Sjjace) in the hud. 



This principle the author has before enunciated, with regard to the 



