In this sentence, Brother uses a natural metaphor and a metaphor of life and death. What he's attempting to show through these metaphors is the destructive nature of pride. Brother's so full of pride in trying to get Doodles to walk again that he's lost sight of what is in Doodles's best interests. Brother wants Doodles to walk so he won't have to be embarrassed by him and his weak body. It's only later on, when Doodles dies from being driven too hard by Brother, that Brother realizes his pride was like a seed that bore two vines: life and death. Brother's pride led to Doodles being given a new lease of life, as he gradually began to overcome some of the limitations imposed by his disabilities. But tragically it also led to his death, as ultimately Doodles's weak body was unable to handle such intense physical exertions.