77 A SURVEY OF THE MISTLETOE OF NEW SOUTH WALES. By Valekik May, M.Sc* (Plate vi; nineteen Text-figures.) [Read 30th April, 1941.] Intro cluction. In recent years considerable interest has been aroused by the apparently increasing incidence of Mistletoe (species of the Loranthaceae) on trees in New South Wales. Where infestation is heavy there is little doubt that the economic effects are serious. Fruit and timber trees as well as ornamental and shelter trees are all liable to attack by the members of this group of hemi-parasites. The germination of the seed, development of the young plant on the host and penetration of its haustoria have been discussed by McLuckie (1923) and need not be considered here. When the parasite has become established on a branch of the host it apparently cuts off supplies of water and mineral salts from the parts of the branch beyond the point of infection. Loss of foliage occurs and often this part of the branch is so injured that it dies and is shed, the Mistletoe thus assuming a^ terminal position. Because of this the host plant may develop a straggling habit, lack of symmetry ' caused by the parasite sometimes being extreme. The quality of the timber may be affected adversely by the swellings at the junction of host and parasite; this is of greatest significance when the infection is on the bole itself. Several reports have been received of secondary infection by fungal and insect pests through the gap in the host tissue left by the death of a Mistletoe. Even where the trees are of too poor a quality to be of economic importance as timber, their destruction by the parasite may leave the way open to increased soil erosion and consequent depreciation of land values. Parasitized plants have also been reported to have a lower production of flowers, pollen, honey and fruit. It has also been reported that if trees with and without Mistletoe are felled, the healthy ones give ris^ to numerous suckers, but the infected trees do so very rarely. Field observations indicate that the presence of Mistletoe leads to a reduction in the rate of growth of the host tree. As a result of this it assumes a ragged appear-ance, and, if the tree is not treated, it will finally die. The term "finally" is used advisedly; most observers quote about twenty years as the time needed for Australian Mistletoes to kill a host, although others quote as short a time as six months. The time varies according to the species of host and parasite, age of host, the conditions of growth and the number of infections present. No experiments in Australia have been recorded which give actual measurements of the reduction in growth rate of the host, but it is of interest to note the observations of workers in other countries where Mistletoe is a pest. Boyce (1925) says: "In eastern Oregon ... it was found that the height of infected 100-year-old yellow pines was 36% less than normal individuals of the same age, while diameter growth was reduced 17%. These figures for Douglas fii's of the same age were 15% and 20%, while for the western larch they were 45% and 41%." Nuessle (1930), from Germany, reports observations on five similarly situated branches of a medium-sized red fir tree. * This paper wa.s prepared when the writer held a Linnean Macleay Fellowship in Botany.