For the past 15 years, there has been tremendous and ever-increasing industrial and academic interest and technological material development concerning plant fibre reinforced composites.Plant fibres can be sourced from a multitude of natural agro-sources, with the preferred choice as a composite reinforcement material often being driven by abundance, geographical location, and historical use. While from a product designer's or engineer's point of view, all plant cell walls are 'similar', there is indeed substantial diversity in plant fibre cell walls. Indeed, a growing body of research demonstrates how cell wall mechanical behaviour is strongly linked to its *Manuscript structure, biochemical composition and the plant growing conditions. At the same time, significant progress has been realised on the knowledge of plant fibre cell wall reinforcement mechanisms in biocomposites. Here, we provide a holistic overview of the main types of plant cell walls used as reinforcements for polymer composites. By synthesising a large body of bibliographic data, we present and discuss the useful diversity in cell walls, illustrated by original schematics for clear comparison. The link between their structures and main properties, in constant link with potential associated composite, is specifically discussed. Then, the different fibre extraction and cultivation modes are discussed and compared, especially through an environmental assessment. We also show how a scientist's point of view on cell wall structure and associated experimental approach and use of characterisation techniques lead to distinct results; following a critical review, we make recommendations on appropriate characterisation techniques for a specific fibre. A final discussion concludes this work by highlighting the pertinent parameters that accurately define a composite reinforcement cell wall. The review will serve as a useful handbook reference for researchers in the field of bio-based materials, and will provide important insights to designers and engineers for appropriate selection of plant fibre cell walls for specific composite applications.

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The biocompostability of natural fibre-reinforced biopolymers, also known as biocomposites, makes them attractive alternative to glass fibre-reinforced petrochemical polymers. The aim of this work is to study the capacity of flax/PLLA (poly(l-lactide)) biocomposite (20% and 30% fibres by weight) to be recycled. Mechanical properties were evaluated initially, and shown to be similar to those of glass/PP and superior to hemp/PP and sisal/PP composites. Then after repeated injection cycles tensile properties were shown to be conserved until the third cycle. Matrix degradation and fibre aspect ratio were followed using molecular weight measurements, thermal and rheological analyses, image analysis of sections and SEM fractography. These techniques revealed a lower molecular weight, lowering of glass transition temperature, reduction of fibre length, and separation of fibre bundles with injection cycles. Nevertheless, the property retention after three cycles under extreme recycling conditions (100% recycling with no added virgin polymer) indicate the promising recyclability of these materials.

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