Darwin is describing crops on a farm:
"Mandioca or cassada is likewise cultivated in great quantity. Every part of this plant is useful; the leaves and stalks are eaten by the horses, and the roots are ground into a pulp, which, when pressed dry and baked, forms the farinha, the principal article of sustenance in the Brazils. It is a curious, though well-known fact, that the juice of this most nutritious plant is highly poisonous."
The "mandioc or cassanda" is evidently Portuguese for
manioc or cassava" a kind of winter melon. I was surprised to find that "farina" is not always ground cereal grain:
farina
n.
5ME < L, ground grain, meal < far, sort of grain, spelt < IE base *bhares3 > OE bere, BARLEY6
1 flour or meal made from cereal grains (esp. whole wheat), potatoes, nuts, etc. and eaten as a cooked cereal
2 potato starch or other starch