jgS MORPHOLOGY OF MEMBERS, 



words : — New lateral members have their origin above the centres of the ividest 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 Char a, Salvinia, the flowers of Reseda, kc, 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 belong 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 bi-anches 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 Muscinese 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 \. When the segmentation of the apical cell is into 



* Lorentz, Moosstudien. Leipzig 1864. 



