1. Rosa Sabini and R. Wilsoni are reciprocal crosses between R. pimpinellifolia and some Tomentosa microgene.

2. Rosa pimpinellifolia is a balanced tetraploid, both the egg cell and the generative nucleus possessing 14 chromosomes.

3. The Tomentosa microgenes are unbalanced pentaploids, the microspores, when functional, carrying 7 chromosomes in their nuclei, and the egg cells 28.

4. Reciprocal crosses between R. pimpinellifolia and tomentosa forms should not therefore agree in chromosome complements. With pimpinellifolia as seed parent the cross should have 14 + 7 ( = 21) in its somatic nuclei, and with the same plant as pollen parent the number should be 28 + 14 ( = 42).

5. R. Wilsoni undoubtedly has R. pimpinellifolia as seed parent, yet its chromosome number is 42.

6. It has, therefore, like Primula Kewensis, doubled its original complement.

7. In doing so, again like that hybrid, it has attained fertility.

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