MISTLETOES
[All images are clickable, for a larger version]

Several large tawny brown clusters of desert mistletoe (Phoradendron californicum) on a paloverde tree in the Sonoran desert

All mistletoes produce fleshy, succulent berries which are eaten by birds to obtain the viscous, sugary flesh. In many cases the bird scrapes the seed onto a branch or other object, or the seed is voided with the faeces after a few minutes. In this way the seeds can become lodged on the bark of a host tree. The seeds germinate spontaneously if exposed to light - usually they need no other germination trigger. The structure that emerges from the seed is termed a hypocotyl - a green stem-like structure with a rudimentary root at its tip. This structure elongates and penetrates beneath the bark of a tree. Then it produces a special nutrient-absorbing structure termed a haustorium, which establishes close contact with the conducting tissues of the host and will eventually tap the host's water and nutrients.

The mistletoes have green, photosynthetic tissues, so they can obtain at least part of their sugars and other organic nutrients from photosynthesis. But they depend on the host for their supply of water and mineral nutrients, and so they inevitably extract some toll on the host's resources. Parasites of this type often are termed hemiparasites (as opposed to holoparasites which depend on the host for all of their nutrients).

Left: single large cluster (about 1 metre long) of desert mistletoe on paloverde. Right: Part of a mass of stems with fruits of desert mistletoe. The scale-like leaves are inconspicuous.

An interesting feature of mistletoes in general is that they can have quite broad host ranges, but with varying degrees of host specificity. For example, the desert mistletoe (Phoradendron californicum) typically infects nitrogen-fixing trees in the family Leguminosae, such as mesquite, paloverde, ironwood and acacias. It usually produces a tangled mass of narrow, branching stems - sometimes green but usually tawny brown, with small, scale-like leaves. It is an evergreen plant, which eventually attains such a size that it kills the major branches of its host, or even the whole tree.

Swollen and cracked part of a branch of paloverde, where desert mistletoe branches have emerged

A different type of mistletoe, called Phoradendron flavescens, has broad, rounded and fleshy leaves. It is found at higher elevations in the desert, and is quite common elsewhere in south-western USA. It parasitises a different range of plants, such as cottonwood (in the poplar family), willows and sycamores.

The broad-leaved mistletoe, Phoradendron flavescens, parasitising a shrub in the higher elevations of the Big Bend National Monument, Texas

Yet another type of mistletoe, Phoradendron bolleanum, is a common parasite of juniper and cedar trees.

P. bolleanum on juniper in an upland region of Texas

Another interesting feature of mistletoes is that their seed dispersal is limited by the distance that birds travel (and the short retention time of the seeds in the gut). Thus, mistletoe-infected trees tend to be seen in patches. Also, it is quite common for a single tree to have multiple infections, if birds eat the mistletoe berries on that tree and scrape the seeds onto another branch of the same tree. The first image on this page shows this clearly.

P.tomentosum, the Christmas mistletoe, is parasitic on a range of woody plants that grow in the higher elevations of deserts. Like the common mistletoe of Britain and Europe, it has broad, yellowish-green leaves and produces white berries. The species name "tomentosum" refers to its slightly hairy leaves.

Juniper mistletoe

Juniper foliage and young fruits (left), and swollen club-like leaves of the juniper mistletoe