Oil glands and ducts in Eucalyptus L'Hérit. I. The phloem and the pith

Abstract

Most (94 %) of the known eucalypts have been examined for the presence of oil glands in vegetative tissues other than the mesophyll and the primary cortex of the stem.

In certain species oil glands occur in the pith and, in a few cases, in the midrib as well. In others (series Corymbosae) oil ducts occur in the pith and in the midrib. In others again (Macrantherae Normales) glands are formed in the phloem of both root and shoot. As a general rule, species which have glands or ducts in the pith do not have glands in the phloem, and vice versa. The only exceptions are provided by nine species of the "eudesmioid complex" (defined below). Of eucalypt species, 58 % have no glands in either the pith or the phloem. It is believed that glands never develop in the primary cortex of the root but always in that of the shoot. The occurrence of glands in the pith or the phloem is constant in many species but others show great variability in the numbers of glands present. Possible explanations for the observed variability are suggested and further lines of investigation are indicated. The age of the tree at which glands first appear in the bark varies widely from one species to another and less widely within species; glands appear precociously in root bark. The age of the secondary phloem itself at which it produces glands also shows wide variations between species. In the Corymbosae the full development of the oil duct system is closely correlated with the ontogenetic juvenile-adult foliage change. The taxonomic implications of the presence or absence of glands in the pith and phloem are discussed. The characters have proved useful for field and herbarium identification. The presence or absence of pith glands is particularly valuable in dealing with Western Australian species.