MORPHOLOGY OF THE VEGETATIVE SHOOT INEPACRIDACEAE

Abstract

According to Sinnott (I9I4), the Epacridaceae are characterized by the possession of multilacunar nodes, and differ in this respect from all other Ericales, the Clethraceae, Pyrolaceae, Ericaceae, and Diapensiaceae being constantly unilacunar. Bailey & Sinnott (I9I4) quote the family as an example of the association of sheathing leaf-bases with multilacunar nodes. As Hasselberg (1937) has pointed out, the work of these authors loses much of its value by their failure to give the names of the genera and species which they examined. On referring to the account given by Drude (I889), it appears that sheathing leaf-bases occur in only five out of twenty-one genera. The habit of the majority of the family is thoroughly ericoid, which would lead one to expect that the nodes would be unilacunar. I am indebted to Mr R. H. Richens for the information that he had personally observed Epacris ardentissima and Styphelia richei to be unilacunar. In view of the obvious desirability of a more extensive investigation, material was obtained representing twenty-one species belonging to fifteen genera. In this material it has been possible to distinguish three clearly defined types of leaf-base. Typical unilacunar nodes, with narrow leaf-bases, the leaf-scar occupying, at most, about a third of the circumference of the stem, are found in Lebetanthus americanus, Lysinema pungens, Epacris impressa, Styphelia triflora, S. adscendens, S. distans, S. parvifolia, S. acerosa, Monotoca lineata, Acrotriche serrulata, Brachyloma sp., Pentachondra involucrata, Trochocarpa thymifolia, Needhamia pumilio, and Oligarrhena micrantha. A shoot of this type is shown in Fig. I, which represents a species of Styphelia. The same type of structure may be expected to occur also in the genera Prionotes, Woollsia, Archeria, and Conostephium. In Cosmelia rubra (Fig. 2), Sprengelia aristata, and S. incarnata, the base of the leaf embraces the stem, the width of the insertion being actually greater than the circumference of the stem, so that the edges of the leaf-sheath overlap, as in many monocotyledons (Fig. 4). In view of this arrangement it is rather surprising to find only a single foliar trace. Immediately above its point of entry into the leaf, this bundle gives off two branches which run almost horizontally round the base of the sheath. From these girdling bundles spring a number of smaller strands which together form the parallel venation of the leaf sheath and limb. The leaves in these two genera are shed in a very peculiar manner. Separation takes place in the region of the pericycle, so as to cast off not only the leaf but also the entire cortex of the internode below. At this stage the leaf, with its attached frill of stem cortex, can be slipped freely up and down the stem. Owing to the rigidity of the overlapping sheath a leaf may remain in this condition for some time, but a slight lateral pull will suffice to remove it. The collar of stem-cortex is relatively flimsy, and breaks at a touch,