Chromosome studies in the Scilleae

Summary and Conclusions

The chromosome types of three species,Hyacinthus orientalis,Bellevallia romana andScilla nutans, have been examined at various stages; the inter-relation of tlie complements does not point to any affinity between the species or to any common descent from a three-chromosome ancestor. The complement of the Hyacinth is not capable of a purely tetraploid interpretation because one of the four long chromosomes has a definite and constant second constriction while the other three are alike in every respect.

The condition of the constrictions in the three species was followed at different stages: certain constrictions are permanent, others appear at one stage but may be suppressed at another. The phenomenon seems to be the result of a local discontinuity which occurs in different chromosomes at different positions—bisecting the chromosome or cutting off a minute element, which will form, in the contracted state, a “satellite.” Reasons have been shown for believing that the responsiveness in the chromosome to the attraction of the poles is more diffuse than is usually imagined and that this zone of response need not coincide with a constriction.

The appearance of the constriction in all cases may be accounted for by supposing that two chromosome elements are joined by a fine single, or double, chromatin thread surrounded, no doubt, by the film-coat that envelopes the whole chromosome.

Secondary splitting of the chromosomes in the first microspore division was noticed at all stages inHyacinthus orientalis, and inScilla nutans in prophase.

Counts have been made of chromosome numbers in the pollen grains of triploid Hyacinths: various theoretical explanations of the proportions in which they occur have been tested and the results have been found to agree closely with the assumption that each of the extra chromosomes undergoes a constant chance of loss, of the order of one-tenth. The occurrence of somatic variations in chromosome number was noted in heteroploid varieties ofHyacinthus orientalis and inScilla nutans.