  Following a recent work by C. Bai, O. Bellier, L. Guo, and X. Ni
(arXiv:1106.6080) we define what is a dendriform di- or trialgebra in an
arbitrary variety Var of algebras (associative, commutative, Poisson, etc.). We
prove that every dendriform dialgebra in Var can be embedded into a
Rota---Baxter algebra of weight zero in the same variety, and every dendriform
trialgebra can be embedded into a Rota---Baxter algebra of nonzero weight.
