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Two botanical forms of L. hirsutum, 1957;Lewis, 1954), whereas that of unilateral incompatibility has been given little attention. Stout (1952) found stepwise unilateral incompatibility among Petunia integrijolia, P. axilleris, and P. parodii. The first two species listed are self-incompatible. Their FI hybrids were all self-incompatible, but 53 of 97 plants were cross-compatible with both parent species, except where S alleles interfered. The remaining plants were of mixed or ill-defined compatibilities. According to Stout, "numerous irregularities were found, especially in the cross-fertile relationships between mating groups." Some of the 46 irregular plants were unilaterally or bilaterally incompatible with the parent species. McGuire and Rick (1954) found self-incompatibility and unilateral incompatibility in the F I hybrids of Lycopersicon esculentum X L. peruvianum. Of the F 2 hybrids, all were unilaterally incompatible towards L. esculentum, and 28 of the 31 plants tested were unilaterally incompatible with females of L. peruvianum. All F 2 plants were self-incompatible. In backcrosses of a type of self-incompatibility from L. chilense to L. esculentum, Martin (1961b) found complete association of self-and unilateral incompatibility. The genetics of unilateral incompatibility has not been studied in crosses of self-fertile species.The studies reported herein were made to investigate further the occurrence and distribution of unilateral incompatibility in L. hirsutum, and to relate the information to the occurrence of other crossing barriers.Self-incompatibility is the inability of a fertile hermaphroditic plant to produce seed through self-pollination, whereas unilateral incompatibility is the ability of two fertile hermaphroditic plants to produce seed in only one of the reciprocal crosses. The relation between the two phenomena is not clear. Lewis and Crowe (1958) postulated that the ability of self-incompatible plants to inhibit pollen tubes from self-fertile plants is a property of incompatibility of S alleles. Rare self-fertile types that cross bilaterally with self-incompatible species were said to be intermediates in the evolution of self-incompatibility to self-fertility. These appear to have lost the properties of specificity and self-incompatibility, but have retained the property of unilateral crossing. Grun and Radlow (1961) speculated that unilateral incompatibility is a barrier evolved to protect self-incompatible species from the deleterious effects of crossing with sympatric self-fertile types. Consequently, such a barrier would not be expected between types that had never been sympatric. A complex unilateral incompatibility found in Lycopersicon by Martin (1961a) further complicates interpretation. The incompatibility occurred in an orderly stepwise fashion among four lines of L. hirsutum and one of L. esculentum. In a recent study, Pandey (1962) found unilateral incompatibility between self-incompatible species as well as unexpected reciprocal compatibilities and incompatibilities i...

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