XVI. On the leaf-arrangement of the crowberry ( empetrum nigrum )

Abstract

Pursuing the study of leaf-arrangement, the author finds that the crowberry of our moors ( Empetrum nigrum ) habitually exhibits a peculiar mole of variation in the arrangement of the leaves on different parts of the same twig. Out of fifty crowberry-twigs taken at random, only four (and these fragments) preserved the same arrangement throughout. In the remaining forty-six the leaf-arrangement was found to undergo a progressive change in ascending from the base of the twig to the summit— a change from a simpler order to others more complex. In general the basal order was that denoted by the fraction ⅖; and this was found to pass most frequently into 2/7, which in turn was found to pass into 2/9, with or without an intermediate set of whorls of 4:2/9 generally passed into whorls of 5, sometimes into 2/11, which was the most complex arrangement that was met with in this plant. The following is a list of the transitions found in the fifty specimens:— In all these instances the striking peculiarity to be observed is that the arrangement passes from an order belonging to one phyllotactic series ( e. g. from the order ⅖ in the primary series ½, ⅓, ⅖, &c.) to an order belonging to another phyllotactic series ( e. g. to the order 2/7 in the secondary series ⅓, 1/4, 2/7, &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 ⅖ to ⅜, 5/13, 8/21 &c.), and is such as would result from uniform vertical condensation of the lower arrangement (as the author has shown in a paper read before the Royal Society on the 30th April, 1874: see Proc. vol. xxii. p. 298).