Odd multiple polyploids are relatively infertile, consequently in fruits such asRubus and.Prunus, where seed and fruit development are closely associated, triploid and other odd multiple forms are relatively unproductive. Triploid apples however are productive,e.g. Bramley’s Seedling, a triploid, is probably more widely cultivated in this country than any other apple.

In apples a very low proportion of fruit to flowers is sufficient to give a yield. The apple has ten embryos, and often a single seed is sufficient for the development of a fruit, and even this seed may be imperfect. This approaches parthenocarpy and renders fruit production still less dependent on the formation of seeds. Fruitfulness in apples may therefore be maintained in spite of a high degree of generational sterility.

Triploidy in apples is another example of the occurrence of sterile forms in species where a substitute has been found (either in nature or in cultivation) for normal seed and fruit production. The substitution in apples is more complex than usual for, while sexual reproduction is now replaced by grafting, the necessity for the stimulus of seed growth in the formation of a fruit is largely evaded. Therefore triploids are able to fruit although incapable as a rule of yielding offspring of any value.

The offspring of triploids, whether derived from selling or crossing with diploids, lack vigour, presumably owing to their aneuploid constitution (cf. Darlington and Moffett). Consequently triploid varieties are likely to be of little value in practical breeding as the necessary vigour and fertility would rarely be obtained in the resulting offspring.

An account is given of the flower colours, chromosome numbers and flower pigments of cultivated species and hybrids ofStreptocarpus.

The origin of the garden strain is recounted and the results of breeding experiments within the strain are shown to agree with the known origin of the garden forms.

The action of the flower colour genes is as follows.A is necessary for the production of anthocyanin, and alone, produces anthocyanins derived from pelargonidin.R in the presence ofA produces anthocyanins derived from cyanidin.O, is epistatic toR and in the presence ofA gives anthocyanins derived from delphinidin. ThusR adds one substituent hydroxyl (or methoxyl) group to the anthocyanidin molecule andO adds two.D produces 3 : 5-dimonoside in place of pentoseglycoside-dimonoside (d) mixtures. Seven distinct colour classes results from the combination of these genes. Methylation is complete with the delphinidin derivatives but incomplete with the cyanidin derivatives.

When the two dominant genesL andE occur together inStreptocarpus Rexii, they give rise to abnormal foliar growth, though flowers and seeds, when produced, are normal. These genes have not been found in twenty other species of the genus. Both occur in wild populations andE has also been found in the garden hybrids derived from species crosses withinRexii some 50 years ago.

Mutations and chimaeras are described. One of the latter gave rise to tetraploids, the first to be recorded in gardenStreptocarpus.

It is considered doubtful whether these genes can play a part in speciation.