Not only the chemical components but also the physical conditions would affect self-bonding. As shown in Fig. 5(a), there were many trichomes and wart-like protuberances on the epidermis of rice straw, and they were possible to influence self-bonding, because the trichomes and protuberances were still present after hot-pressing (Fig. 5(b)). Moreover, their size and number were more or less the same as before pressing. It is clear that trichomes and protuberances were not flattened during the process of board manufacture. Accordingly, almost all surface areas of particles could not contact directly with each other, owing to the presence of the trichomes and protuberances. This suggests that trichomes and protuberances would inhibit the self-bonding between particles.