"Why should the flower be so extraordinarily big and out of proportion to the rest of the body , the threads hidden within the tissue of the vine ? Its wide spreading petals cannot assist in the dispersal of its scent, as the tail spadix of the titan arum almost certainly does.... Plants, like any other living things, are ruled by cost-efficiency budgets. The food and energy they expend in growing flowers, leaves or any other organs must normally bring a commensurate advantage of some kind. But Rafflesia may not be so curbed. It does not, after all, earn the food it expends. *It takes it straight from the vine. Provided that the vine is not weakened by the loss to the point of death, there seems to be no limit to the amount the Rafflesia may extract and therefore no limit to the size of the flower it constructs. Maybe an unearned income in the plant world, as elsewhere, can lead to profligacy and extravagance on a truly monumental scale."
*Rafflesia is a parasite on Tetrastigma vine.
From "The Private life of plants"