Abstract. Unfortunately the class of transformations computed by nondeleting and linear extended top-down tree transducers [Graehl, Knight: Training Tree Transducers. HLT-NAACL 2004] is not closed under composition. It is shown that the class of transformations computed by nondeleting and linear bimorphisms actually coincides with the previously mentioned class. Moreover, every nondeleting and linear bimorphism with an -free input-homomorphism can straightforwardly be implemented by a multi bottom-up tree transducer [F