198 
MORPHOLOGY OF MEMBERS. 
words : — New lateral members have their origin above the centres of the widest gaps 
which are left at the circumference of the growing point between the insertions of the 
nearest older members of the same kind. The rule is illustrated by the case of alterna- 
ting whorls (especially of pairs crossing one another at right angles, or 'decussating 
or by that of distichous leaves with a base which grows early in breadth, in 
Phanerogams where the growing point consists of small cells. Where, on the other 
hand, we have decidedly bilateral horizontal axes, as in Pieris aquilina, Salvinia, and 
Marsilea, or definite relations of the phyllotaxis to the segmentation of an apical 
cell as in Mosses, or distinctly successive formation of the members of the same 
whorl, as in Chara, Salvinia, the flowers o^ Reseda, &c., the mechanical importance 
of the rule is, in my opinion, subordinate to the other causes which then have the 
greatest influence in determining the position of the new members. Independently 
of the points of view referred to in paragraphs 1-4, the genetic relationships indicated 
in this paragraph show that it is scarcely possible to find a single rule which will 
govern all cases of phyllotaxis. Causes which belang to altogether different cate- 
gories must, according to circumstances, exercise the greatest influence in determining 
the point at which a new member is formed. 
(6) I consider it a circumstance of primary importance that the same or very 
similar kinds of phyllotaxis may be brought into existence by very different com- 
binations of causes, and arrangements apparently very different by very similar 
combinations of causes. Among the causes here referred to I include the anterior 
development of the axis and of its lateral members, the influence of the primary on 
the secondary axes, the effect of pressure, gravitation, light, and similar conditions. 
This position becomes evident when it is- observed that the same or similar diver- 
gences of leaves or lateral shoots may occur everywhere, in unicellular plants, in 
multicellular plants with a distinct apical cell, and in those in which the growing 
point consists of a small-celled tissue without any definite relation to the segmentation 
of an apical cell, as in Phanerogams. The mechanics of growth must undoubtedly 
be different when the lateral branches of the single cell of Vaucheria are formed in 
two rows, and the leaves of a Fissidens or of a Grass are produced in the same or 
a similar position, in which case the cell- walls of the primary meristem represent a 
multiplicity of causes of growth and of hindrances to it. The similar arrangement 
of the outgrowths under such different circumstances does not prove that the cir- 
cumstances themselves are indifferent, but only that altogether different combinations 
of causes may lead to very similar relationships of position. In Muscineae and 
Vascular Cryptogams the relation of the formation of leaves to the segmentation 
of the apical cell is the more obvious the nearer the leaves originate to the apex. 
It is most obvious of all in Mosses, where each segment grows out into a leaf- 
forming protuberance as soon as it is formed, and before further cell-division 
takes place. Here the immediate controlling cause of the position of the leaves is 
that of the leaf-forming ' segments ' themselves ; when these latter are formed in two 
alternating longitudinal rows, as in Fissidens \ two rows or orthostichies of alternating 
leaves arise with the divergence J. When the segmentation of the apical cell is into 
^ Lorentz, Moosstudien. Leipzig 1864. 
