This ethnoecological study of the Makushi Amerindians of Amazonian Guyana explores the place of plants in the indigenous culture and cosmology. The North Rupununi, the homeland of the Makushi people, is a bioculturally diverse mosaic of neotropical savannahs, forests, and wetlands. As subsistence hunters, fishers, and horticulturalists, the Makushi live in a constant and dynamic interaction with their ecologically rich surroundings. Against the human-faunal bias latent in much Amazonian anthropology, I place plants firmly at the centre of analysis, a positioning that mirrors their centrality in the ethnographic context. Human-plant encounters explored herein include swidden agriculture, the cultivation of bitter cassava, the fermentation of cassava drinks using a domesticated fungus, the use of a category of charm plants, and the consumption of plant substances in shamanic ritual. With the Makushi, I emphasise the status of plants as living selves and agents of semiosis, occupying perspectives on the world in and outside of their interactions with human beings. In order to investigate ethno-theories of life, I attempt to understand the constitution of the person - and associated notions of body and soul - in the indigenous cosmology. Makushi ontology can be characterised as animic - though as I argue, it also incorporates naturalistic and analogic elements. Thus, it is poly-ontological. This study pursues a dual goal: first, to pay heed to the trans-specific domain of living entities revealed in the Makushi ethnoecology, and second, to rethink conventional symbolic frameworks characteristic of anthropological approaches to culture. I explore the application of a more robust approach to sign-flows in nature - Peircian ecosemiotics - that allows for the analysis of plant communication, birdcalls, insect stings, and leaf patterns, as well as human language. In tracing these interspecific webs of signification, conclusions are drawn about the varied ways in which Makushi people engage with and think about their living environment. At the same time, many Makushi multispecies engagements are based on the physical transfer of substances between bodies of different kinds. In order to better account for this pervasive 'substance logic', greater attention must be paid to indigenous notions of corporeality and personhood. In doing so, I propose a dual analytical model that takes both the flows of signs and the flows of substances as its combined objective. This approach enables new conclusions to be drawn about multispecies relationality in indigenous Amazonian cosmologies.