SUMMARYThe major C14‐labeled substance in sieve tube exudate ofM. pyriferaisD‐mannitol, comprising 3.6% (w/v). No sugars are detectable. Certain amino acids also possess some C14‐labeling and occur in significantly high concentrations in exudate. The exudate contains negligible ether‐soluble lipid, but has a large amount of protein and a high concentration of K+Neither protein nor lipid become labeled significantly in sieve tubes during short‐term translocation experiments with C14. In general the chemical composition of the assimilate stream is comparable to that of vascular plants and does not, consequently, necessitate a different mechanism for tran