The large brightly coloured networks of tube we see are the adult form of a slime mould - called a plasmodium but they have several stages in their lives.
Spores - where it all starts
Slime moulds have a primitive form of sexual reproduction. The nuclei in plasmodia are diploid - they have two sets of chromosomes. If the organism is exposed to light for a few days, the plasmodium clumps together and forms short stalks with minute mushroom like caps. At the end of these stalks nuclei undergo meiosis - chromosomes are reshuffled and nuclei divide making spores which are haploid - just one set of chromosomes. These mushroom like fruiting bodies are called sporangia.
Making sporangia (spore capsules) is fatal for a slime mould - the end of the adult organism but its spores are dispersed by wind and germinate when they land somewhere damp. The spores hatch into amoebae (strictly speaking they're myxamoebae). Under the microscope they're indistinguishable from ordinary amoeba - they feed by engulfing bacteria and they reproduce by splitting in two. If things get dry, they form a cyst that can withstand dehydration until conditions are better and if things get too wet, they grow flagella (tails) and switch to a free-swimming cell form.
They can live independently like this indefinitely, it's an extreme form of the alternation of generations we see in ferns and mosses. The amoeba are actually gametes - sex cells. Most species on earth have two different sizes of gamete - one large and immobile - the egg or ovum and the other smaller and mobile - sperm or pollen. This isn't the case for a slime mould, all their sex cells are the same size and shape, this is called isogamy. What determines if they’re able to fertilise each other is a set of genes that form the cells mating type – the average slime mould can have hundreds of different mating types maximising the potential number of mates.
Plasmodia - Grown up Slime
When two amoebae with compatible mating types meet their cells merge and their nuclei fuse – going from being haploid to being diploid. Once this happens the cell stops dividing but the nuclei do – the cell just expands and forms a new plasmodium . In some cases, a single plasmodial slime can cover a few square metres.
Most slime moulds live in leaf litter, rotting wood or soil - damp places, they're very vulnerable to drying out but it isn't always a problem for them.
Plasmodia can tough it out by forming a sclerotium ( sclerotia is the plural ). This is a hardened mass of tissue that has dried completely and can survive for years. If it gets wet, it reactivates and the slime mould resumes its former life.
Amoebae and swarmer cells that start to dry out will thicken up their cell wall and form a cyst - a tough single celled structure that withstands drying and will wake up when it gets damp again.
Swimming flagellate cells