No abstract

The vegetative axillary buds of Eucalyptus regnans F. Muell. at various ages were studied by light microscopy in serial sections and by direct observations in the field and glasshouse. All buds (except the very first apical bud) originated from axillary meristems, i.e. from generative tissue which arose in the axils of primordial leaves and survived in a meristematic condition for many years. Each axillary meristem normally produced one emergent primary bud and then an indefinite sequence of concealed accessory buds. The extensive dynamic shoot-system condensed within a primary bud comprised secondary as well as tertiary axes and their respective appendages. All parts were present throughout the year in a continuous sequence of maturation which extended also to the expanding shoot. During winter, development appeared to be merely slowed down or suspended. Primary buds which did not grow into shoots were shed after only a few weeks. The accessory buds were formed in a uniserial descending series at the base of and abaxial to each primary axillary bud. The first of the accessory buds was initiated within the primary bud, and the second within the expanding shoot. The first accessory bud resembled young primary buds in structure, but subsequent accessory buds were less and less complex. Keeping pace with the cambium, the axillary meristem formed a radial trace of thick-walled parenchyma in the wood and accessory buds embedded in a strand of thin-walled parenchyma in the bark. The distal portions of the bud strand and the buds embedded in it were shed progressively with the decorticating bark. Each of the bud strands which traversed the bark of 20-year-old E. viminalis Labill. was found to contain six to 12 radial strips of meristematic tissue. When epicormic growth was stimulated, several of these strips produced files of separate, new, condensed shoots. Of the scores of shoots thus initiated throughout the length of the bud strand, up to 10 or 20 of the distal ones emerged from the bark and grew into epicormic shoots. The buds of 20 other eucalypt species were examined by dissecting microscope only. It appeared that their bud systems were essentially similar to that of E. regnans. The widened concept of the axillary meristem shifts attention from individual buds to the continuous generative powers of the axillary meristem and helps to explain the outstanding capacity of the eucalypts to produce new shoots.

Permanent milacre quadrats were stsdied on a mnge of sites burnt in vario.;s years. All the recent burns occurred in March. Low herbs, predominantly bryophytes, colonized about 90% of the ground 12 months after burning, and 99 % 6 months later. Marchantia polymorpha was one of the earliest colonizers. On the wetter sites it covered up to 75 % of the ground 14 years after burning, and then declined rapidly. Marchantia growing in exposed places tended to be killed during summer. The fire mosses, Funaria hygrometrica and Ceratodon purpureus, colonized most of the ground not covered by Marchantia and reached peaks of 30-84% cover about 2 years after the fire. After 5 years the fire mosses had almost disappeared. Polytrichum juniperinum succeeded Marchantia and the fire mosses, reaching its peak (38-90 %) about 4 years after burning. Practically all of the low herbs tend to disappear when the taller vegetation becomes dense. The taller vegetation tends to be successively dominated in time by herbaceous fire-weeds, ferns, and woody perennials. Senecio minimus, the most abundant fire-weed, reached up to 79 %cover 2 years after burning, but died suddenly after reaching its peak. The development of ferns depended largely on the abundance of rhizomes surviving the fire (abundance before the fire, nature of fire). Histiopteris incisa and Hypolepis rugulosa were typical of sites which had carried rain-forest understoreys. Pteridium esculentum was initially limited to sites which had carried wet sclerophyll understoreys, but tended to replace the other ferns within 10 years on all but the most sheltered, moist sites. Although patchy at first, the ferns covered 50% of some areas within 4 years. Their expansion and persistence seems to be limited mainly by competition from taller plants. Most of the woody plants germinated within a few months after the fire from seed which was either stored in the ground or shed from nearby trees (depending on the species). Availability of seed and intensity of browsing by native game appear to be the two main factors which determined the abundance of established woody plants. Under favourable conditions the woody plants became dominant so early that the fern stage was by-passed. Intensive browsing was able to delay the dominance of woody plants, often for many years.

No abstract