Case Studies

Microporus xanthopus - growth and sporing

The English mycologist Edred John Henry Corner (1906-1996) lived in Singapore between the 1920s and 1940s. During that time he studied the development of the fruiting bodies of various fungi, including Microporus xanthopus, which Corner referred to by another name: Polystictus xanthopus. The mature fruiting bodies have thin, funnel-shaped caps that are concentrically zoned in various shades of brown and which are supported by a yellow-footed stem. The cap, or pileus (plural: pilei), is generally between 1 and 3 millimetres thick. Microporus is a polypore genus. On the white underside of the pileus there are numerous tiny pores (about 10 per millimetre) and this feature is the source of the genus name - literally "tiny pore". The term xanthopus is derived from two Greek words and literally means "yellow foot". The species is found on rotting wood and is common from the Australasian, Asian and African tropics, but is absent from the American tropics.

Corner collected some embryonic specimens and then watched their developments under bell jars, which helped to maintain the necessary humidity. In his experiments the pilei stopped growing after reaching between 9 and 15 millimetres in radius, though in the wild the pilei can grow to over 60 millimetres in radius and Corner noted a specimen of about 100 millimetre radius in the Singapore herbarium.

To the naked eye the initial stage, or primordium , of the fruiting body is simply a white fleck on the wood surface. This enlarges into a hemispherical cushion between half a millimetre and a millimetre wide. Then the primordium elongates via vigorous growth to develop the stem. Eventually the hyphae in the central part of the stem apex stop growing while those on the periphery continue to grow and so develop the funnel-shaped pileus. As well as growing in length the stem has also been thickening and a wider basal foot has also been developing. Generally, the basal foot has completed its development before the pileus has appeared. Stem growth was, on average, 1.3 millimetres per day but the growth rates showed variations, not tied to any particular stage of development. For example, one grew its stem at 2.5 millimetres per day from a length of 7 to 18 millimetres (eventually reaching 34 millimetres after 22 days). Another grew at 1 mm per day during its whole development to 15 millimetres. The pilei grew at the rate of about 1 millimetre per day in radial growth and both the stem and pileus grew as well in day time or night time. Given the growth rates he observed, Corner estimated that a well-developed wild specimen, with a stem of about 25 millimetres and a pileus of 40 millimetres radius would have taken 59 days to develop - 19 days for the stem and 40 for the pileus. But he did caution about extrapolating bell jar experiments to growth under natural conditions, with optimum jungle conditions perhaps allowing faster growth.

The largest specimens were in places that were always damp, such as by watercourses in dark shade. Those in secondary jungle or places which were apt to dry out at frequent intervals and which were continuously humid only for a month or so at a time were always rather small. Corner never saw any evidence of discontinuous growth, noting that the margin never grew afresh in specimens that had dried out, which gives an explanation for smaller specimens in those places likely to be subject to dry periods.

Sporing continues, night and day, for as long as the pileus is growing and sporing continues for some time after the fruiting bodies have stopped growing, by drawing on reserves within the fruiting bodies. Corner collected nine fresh, medium-sized fruiting bodies and, detached from their wood, placed them in a damp atmosphere, occasionally moistening them. Of the nine, two spored for 10 days, one for 35 days and the rest for between 16 and 25 days.

While fruiting bodies that had dried did not resume growing on being moistened, Corner found that he could induce sporing in fruiting bodies that had been dried. He collected about 30 specimens which he allowed to dry out in room temperatures of 75-85 degrees Fahrenheit. He re-soaked them after varying intervals: 5 after 12 days; 2 after 14 days; 2 after 18 days and 4 after 21 days. All these fruiting bodies spored heavily after soaking out. Two further fruiting bodies soaked after 28 days spored less abundantly and three soaked after 35 days spored hardly at all. A number of others re-soaked after 28, 42, 52 and 60 days failed to revive at all but became mouldy. He also dried several fruiting bodies for two or three periods of two weeks, broken up by short periods of sporing. It was not until after the third or, in two cases, the fourth drying that the fruiting bodies failed to revive at all. Corner noted that in cases of repeated drying (as well as in fruiting bodies that had been dried for more than 20 days) the sporing was confined to the area near the margin of the pileus, the other pores seemingly having become moribund.

In detached fruiting bodies which had been sporing for some time the pilei became thinner and thinner until they had become quite papery and pliant when fresh, but brittle and parchment-like when dried. Microscopic examination showed that the normally thick-walled hyphae that are abundant in the pilei of this species, had become "eroded", the material stored in the hyphal walls presumably having been "cannibalized" to promote sporing.

Reference

Corner, EJH. (1932) The Fruit-body of Polystictus xanthopus Fr. Annals of Botany, 46, 71-111 & Plate V.