Library

Notes: Summary The development of characteristic cytoplasmic inclusions from endoplasmic-reticulum (ER) cisternae in nucellar cells of some species of Euphorbia has been studied by electron microscopy. The formation of these organelles is preceded by the appearance of rough ER cisternae filled with an electron-dense material and forming complicated networks. Vesicular structures are formed which grow rapidly to give electron-dense, spherical dilations. On the outer surface of their limiting membrane numerous ribosomes and often polysomes are present. This membrane can be seen to remain continuous with the membranes of one or more cisternae of the rough ER up to when the dilations have a maximum diameter of 2.5–3 μ. At this time, continuity between the ER cisternae and the spherical dilations ceases. After this the new cell organelles remain unchanged in size, shape, and electron-density until the cell is disintegrated by the growing embryo-sac. The fate of the contents of these organelles is discussed.

Notes: Summary Cytological and genetical analyses were made of the breeding system of embryo-cultured interspecific tomato hybrids between L. esculentum and L. peruvianum. It was found that fluorescence techniques and electron microscopy allowed a distinction to be made between pollen tubes inhibited by a unilateral incompatibility reaction and pollen tubes inhibited by a self-incompatibility reaction, after self-pollination of the hybrids or after reciprocal crossing between the hybrid and the parental species. The observed differences, if real and reliable, demonstrate that unilateral incompatibility in esculentum pollen tubes is governed by a single gametophytic factor which is either linked or allelic to the S-locus. This finding is discussed with reference to recent reports that unilateral incompatibility is controlled, in peruvianum styles, by a number of different dominant genes and it is concluded that these dominant genes, the S-locus of self-incompatibility and the gametophytic factor regulating the unilateral reaction in esculentum pollen belong to the same linkage group. The strong sterility barriers which prevent practically all backcrosses between the hybrid and the parental species were shown to be independent of the factors regulating stylar incompatibility. L. peruvianum is heterozygous for the sterility genes which prevent fertilization or embryo formation when the interspecific hybrid is crossed, as pistillate parent, to different accessions of L. peruvianum. One peruvianum stock was found which, as a pollinator, was highly cross-fertile with the hybrids. The presence of a concentric endoplasmic reticulum in inhibited pollen tubes was observed to be a constant feature of both the self- and the unilateral incompatibility reactions and was interpreted as an indication that incompatibility might lead to a general cessation of protein synthesis. Although incompatible tubes very much resemble, in this respect, the pollen tubes cultured in vitro, it seems probable, on theoretical grounds, that the inhibition of pollen tubes in incompatible styles does not result from an absence of growth promoting substances but from the presence of a metabolic inhibitor.