97 AN Fl HYBRID BETWEEN EUCALYPTUS PULVERULENTA AND E. CAESIA. By L. D. Pryoe. (Two Text-figures. ) [Read 30th May, 1956.] Synopsis. A viable Fl hybrid between E. pulverulenta and E. caesia has been produced by manipulated pollination. It is suggested that E. caesia belongs to a natural systematic group with bisected cotyledons and that successful hybridization between this group and many species of the Macrantherae-Normales may be possible. Species which can cross often produce hybrids naturally when they grow near one another. Where they are always widely separated in the field it is necessary to make manipulated crossings to see whether they can hybridize. The capacity to cross is one index (though by no means the only one) of affinity between species (Dufheld, 1952). Text-fig. 1. — Seedlings of (a) E. pulverulenta, (&) E. caesia and (c) E. caesia x E. pulverulenta at about five months. Where each of the pair of species involved in successful crossing is derived from separate systematic groups an indication is given of the affinity of the groups and this in turn makes possible a reasonable forecast as to which species combinations may be successful in a breeding programme.