SPORE DISCHARGE AND DISPERSAL IN MUSHROOMS
Mushrooms are basidiomycetes with numerous basidia on each gill. A
typical basidium is a club-shaped structure, usually with four prongs at one end.
Each such prong is called a sterigma (with sterigmata the plural form) and the
spores develop at the tips of the sterigmata.
Here is a stylized drawing of a basidium, shown in green, with four brown
spores. The colours in this diagram (and in the ones to follow) have no
significance and are simply used to help differentiate the different structures.
Moreover, the diagrams are stylized (rather than being faithful depictions of
actual specimens) but illustrate the important structural features and principles
involved.
Remember that most mushrooms have gills that are V-shaped in cross
section (with the point of the V at the bottom of the gill) and are oriented
vertically. The basidia are held out from the gill surface and protrude into the air
space between two gills. Thus, throughout their development, the spores are
exposed to the atmosphere between the gills. In order to explain the way in which
the spores are ejected from the basidia it is necessary to look more closely at the
spore and its attachment to the sterigma.
In the diagram to the right the black, elliptical outline represents a mature
spore on the end of a sterigma (coloured green, on the left). Many spores are
, smooth and ellipsoid, so this illustrates a fairly common situation - but the
following explanation holds for other spores as well. The red dot indicates the
spore's centre of mass. Note that the spore has a short, blunt, off-centre spike
(called an apiculus or hilar appendage) at one end and the spore is attached to
the sterigma at the apiculus. Every mushroom spore has an apiculus - though there
is some variation in size, shape and orientation of the apiculus between species.
The apiculus-sterigma boundary is a line of weakness and by the time the spore
is mature the link between apiculus and sterigma is very weak.
THE DISCHARGE MECHANISM
Now it is a simple matter to explain the way in which the spore gets off the
gill and away from the mushroom cap. The following diagrams illustrate the first
part of the process (ejection from the sterigma) and the explanations follow.
1. Between the gills the air is still and very humid. At the point of the apiculus
the spore secretes a small amount of sugar molecules. If you've ever left an open
sugar bowl on a kitchen bench for a lengthy time, you would have come back to
find that the sugar had picked up some moisture from the air. Sugar is an excellent
absorber of water vapour. Given the surrounding high humidity between the gills,
water condenses onto the sugar at the tip of the apiculus and forms a drop (the
small, solid, greyish-blue circle). This drop is known as Buller's drop. At the
Mushrooms are basidiomycetes with numerous basidia on each gill. A
typical basidium is a club-shaped structure, usually with four prongs at one end.
Each such prong is called a sterigma (with sterigmata the plural form) and the
spores develop at the tips of the sterigmata.
Here is a stylized drawing of a basidium, shown in green, with four brown
spores. The colours in this diagram (and in the ones to follow) have no
significance and are simply used to help differentiate the different structures.
Moreover, the diagrams are stylized (rather than being faithful depictions of
actual specimens) but illustrate the important structural features and principles
involved.
Remember that most mushrooms have gills that are V-shaped in cross
section (with the point of the V at the bottom of the gill) and are oriented
vertically. The basidia are held out from the gill surface and protrude into the air
space between two gills. Thus, throughout their development, the spores are
exposed to the atmosphere between the gills. In order to explain the way in which
the spores are ejected from the basidia it is necessary to look more closely at the
spore and its attachment to the sterigma.
In the diagram to the right the black, elliptical outline represents a mature
spore on the end of a sterigma (coloured green, on the left). Many spores are
, smooth and ellipsoid, so this illustrates a fairly common situation - but the
following explanation holds for other spores as well. The red dot indicates the
spore's centre of mass. Note that the spore has a short, blunt, off-centre spike
(called an apiculus or hilar appendage) at one end and the spore is attached to
the sterigma at the apiculus. Every mushroom spore has an apiculus - though there
is some variation in size, shape and orientation of the apiculus between species.
The apiculus-sterigma boundary is a line of weakness and by the time the spore
is mature the link between apiculus and sterigma is very weak.
THE DISCHARGE MECHANISM
Now it is a simple matter to explain the way in which the spore gets off the
gill and away from the mushroom cap. The following diagrams illustrate the first
part of the process (ejection from the sterigma) and the explanations follow.
1. Between the gills the air is still and very humid. At the point of the apiculus
the spore secretes a small amount of sugar molecules. If you've ever left an open
sugar bowl on a kitchen bench for a lengthy time, you would have come back to
find that the sugar had picked up some moisture from the air. Sugar is an excellent
absorber of water vapour. Given the surrounding high humidity between the gills,
water condenses onto the sugar at the tip of the apiculus and forms a drop (the
small, solid, greyish-blue circle). This drop is known as Buller's drop. At the
