Many vegan leathers are made from virgin plants or even rescued plant by-products. This could be apples, grapes, pineapples, paper for instance. But in most cases, for a plant by-product to actually act like leather, as in this apple leather example, it needs to be bound with something a polymer (PU). Polymers are mostly synthetic, made from fossil fuels and more commonly known as plastic.
Natural polymers also exist like rubber and wood and also in the root like structures of fungi, called mycelium. The polymer found in mycelium is the same as that found in crab shells. It's not only strong, but it's a bound in a kind of super highway of fibres and that is what makes it an excellent natural leather.
This article on Scientific American explains both the production opportunities for mycelium and the important value of mycelium in the natural environment. In terms of the incredible opportunity mycelium offers production, this is it:
MYCELIUM'S FAST-GROWING FIBERS PRODUCE MATERIALS USED FOR PACKAGING, CLOTHING, FOOD AND CONSTRUCTION. EVERYTHING FROM LEATHER TO PLANT-BASED STEAK TO SCAFFOLDING FOR GROWING ORGANS. MYCELIUM, WHEN HARNESSED AS A TECHNOLOGY, HELPS REPLACE PLASTICS THAT ARE RAPIDLY ACCUMULATING IN THE ENVIRONMENT.
The environmental credentials of mycelium are extensive as it has smallish environmental footprint in the way it grows and easily composts. And then there are the products it replaces - pretty much all have high greenhouse gas emissions in their production and processing, use unsustainable virgin raw materials and don't biodegrade.
When mushroom roots are grown on sawdust, agricultural waste, trays or vertically until they form a thick mat that can then be treated to resemble leather. Dependent upon conditions, it takes a few weeks to literally grow a mushroom leather ready for drying.
Bolt Threads claim to have made the first commercial bag, which they launched on a successful Kickstarter campaign in 2018. The leather is called MyloTM and is grown naturally in precise conditions that create the base mats. It is then harvested, dyed and imprinted with their pattern. MyloTM has since gone on to create ranges for Lululemon, Stella McCartney, Adidas and others.
Uses for mycelium go way beyond leather and companies like European company Mogu have developed acoustic tiles. Mogu Acoustic tiles literally come are made of natural, foam-like mycelium, produced in its beautiful white color with small and unique variations of tone. Traditional paints and varnishes would compromise the biodegradability and overall circularity of the product, as well as its technical properties.