Saturday, December 28, 2013

Vital dust -- a Saturday reprise


That's what Nobel Prize-winning biologist Christian de Duve titled his book on the origin and evolution of life: Vital Dust His title reminds us that we live in a sea of invisible spores swimming on the wind. We breathe these seeds of life in and out with every breath. Rusts. Smuts. Molds. Mildews. Mosses. Mushrooms. Ferns.

Given the quantity and variety of airborne reproductive germs, it might seem likely that our lungs would become gardens of foreign organisms -- the invasion of the body snatchers. But that's not quite the way it works. Most airborne spores must alight in a highly specific environment if they are to bear fruit.

Consider, for example, the cedar-apple rust. Here is a photograph I took yesterday of a mysterious apparition on a cedar tree along my Path -- the tentacled fruiting bodies of cedar apple rust, caused by the fungus Gymnosporangium jumiperi-virginianae.


The life cycle of this curious organism "starts" in the springtime, on an apple tree. Small yellow dots develop on the underside of the leaves shortly after the tree comes into bloom. The yellow spots gradually enlarge and become orange. In late summer, small tubes grow on the lower leaf surface near the orange spots, and brown spots may develop on fruit.

The orange spots release spores that are distributed by the wind. If -- and only if! -- they land on a cedar tree, they germinate and put out tubes that penetrate the tiny leaves. By some chemical magic, these tubes cause the growth of fleshy, reddish-brown galls, called cedar apples, about the size of a grape. The development of the galls and the maturing of the fungus within them require nearly two years from the time of infection.

Then, during wet weather in May, the galls put out the long orange tentacles, slimy and gelatinous, that you see in my photograph. These release spores of a different sort, which make their way by random breezes back to an apple tree. The cedar and the apple are necessary alternate hosts to the fungal parasite. All those spores, at different stages of the fungus's life cycle, wafting back and forth on the wind, utterly dependent upon making a lucky landing on a specific plant.

You'd have to see a cedar tree full of these otherworldly orange-tentacled galls to dream that such a thing could exist. After I had taken my photograph, I stood there on the path shaking my head in astonishment. Vital dust, indeed!


(This post originally appeared in May 2007.)