The life cycle of Flammulina velutipes.

Flammulina velutipes grows on dead or living trunks of various deciduous trees. During mild winters the fruit bodies force their way in bunches through the split bark. The caps have gills on their undersides.

In close-up their surfaces appear to be sprinkled with granules. They are in fact club-shaped cells protruding above the surface. The larger ones, known as cystidia, are sterile. The smaller ones become bacidia.

Four short processes, the sterygmata, grow out of each. At the tip, the sterygmata expand and swell spherically.

Finally, four bacidia spores are formed. The ripe spores are ejected to a distance of about a tenth of a millimetre and drop down between the gills. On a suitable nutrient substrate,

the spores begin to germinate already after about 12 hours. This process can be observed more precisely in the spore to the right. After swelling, the contents come into motion.

The germ tube is growing out. It becomes a thread-like hypha. Now the nucleus enters the hypha and is carried along in the plasma. The hypha branch to form a mycelium.

The hypha grows only at the apex, which contains dense plasma. With increasing length of the cell, its nucleus migrates apically. The plasma behind it vacuulates.

When the apical cell has attained a certain length, the nucleus divides. A new cross wall is formed at the site where the nucleus has been prior to division.

Side hyphae grow out of the subterminal cell a short distance from the new cross wall.

Fungal hyphae can fuse with one another. The apex of a growing hypha meets here a section of an older hypha and fuses with it. The wall is dissolved and plasma constituents change from one hypha into the other.

In the centre of the picture, two short side hyphae of the same mycelium are fusing with their apices. Hyphae of different mycelia can also fuse. On the agar plate, four by two pieces of mycelium lie side by side.

Each of the right ones has the incompatibility factors AXBX. The left pieces of mycelium are different. AYBY, AXBY, AXBX and AYBX.

Hyphae are fusing in the contact zones. If two unequal A and two unequal B factors meet, as at the top of the picture, a dikarion is established. The dikarion has a different growth form.

Its hyphae bear clamp connections. In this general view, arrows mark the places where clamps are to be formed.

The individual cells contain two nuclei, one from each parent monocarion. Clamps are formed in connection with nuclear division. During nuclear division, the right nucleus of the two arrow-marked nuclei

migrates into a hook-shaped protuberance. After division, one daughter nucleus returns to the hypha. The second nucleus has disappeared. It has also divided into two daughter nuclei, but this process couldn't be observed here. Cross walls are laid down in the hyphal sections between the two daughter nuclei.

One of the daughter nuclei remains temporarily in the hook. The process of clamp formation can again be observed in the lower hypha. First, the hook-shaped protrusion grows out.

The two parent nuclei can again be recognised. The right one migrates to the hook and divides.

Meanwhile, the apex of the hook cell of the upper hypha has fused with the hyphal wall. At the point of fusion, the wall dissolves.

Cross walls are being laid down in the lower hypha. In the upper hypha, the nucleus is now migrating out of the clamp through the opened wall.

In mono, as well as in dikaryons, aerial hyphae can break up into oedia. These are short, mononucleate portions of hyphae

which can be easily blown like spores by the wind and thus aid propagation. Once more, oedia formation under greater magnification. To the right, near the apex of the hypha, a cross wall is laid down. Not far to the left, a second one is being formed.

Now, the contents are disappearing between the two cross walls.

Further, oedia are tied off at first near the branch and later in the centre of the picture. The following sequence shows the development of the fruit bodies of Flammulina velutipes.

First, small, compact hyphal aggregates grow out of the mycelium and guttate vigorously. These are initials of the fruit bodies which are also termed primordia. They are more readily formed in dikaryons and only in these do they develop into fruit bodies with basidia.

In this lateral view of a fungal culture, the fruit bodies are growing in bunches. The stipes extend and lift the pileae above the nutrient substrate where they expand and the spores ripen.

The fruit bodies exhibit nutation.

Whereas the development of the mycelium takes several weeks or months, the fruit bodies develop and decay within 10 days.