Mistletoes - cryptic mimicry
As mentioned before (herbivore palatibility and bird recognition), it has been assumed that mimicry does not have a simple physiological basis involving transfer of growth substances from host to parasite. Nevertheless studies have shown that growth substances carried in the xylem sap of the host are in fact taken up by the mistletoe seedling. Some studies also suggest that cytokinins in the xylem sap of host and mistletoe are more similar in mimicking situations than in non-mimicking ones. Even so, it appears that growth substances derived by the mistletoe from the host are metabolized or broken down by the mistletoe, and there is no strong evidence that they directly influence the growth form of the parasite. Although mistletoe plants apparently synthesize their own growth substances, perhaps they can utilize substances derived from the host as precursors. This is a difficult area for research, and further studies are needed.