Extremely stunted seedlings appeared in plantings of PI 188, 565, an Italian variety named San Pancrazio R-15. These seedlings are distinguished from those of all other mutants that I know by their very short hypocotyls, and short, dark, and strongly recurved cotyledons. Growth is very slow, the plants reaching a height of not more than 15-18 inches in an entire year of culture in the greenhouse. All parts of the mature plant are greatly reduced in size and most organs are modified in the direction of shorter and broader shapes. Leaves are very dark green, the surface bullate, and the midrib twisted. Despite these abnormalities and the great modification of flowers, viability and fertility are good. Undisturbed plants in the field survive with normal care and set numerous fruits with seeds during the course of a season.
In F1 hybrids this character behaves as if completely recessive to normal and to d. The segregation in F2 is respectfully 3 normal: 1 extreme dwarf and 3 dwarf: 3 extreme dwarf, a typical observed ratio for the latter being 395 dwarf: 124 extreme dwarf. Since only two stature phenotypes have appeared in the segregating generations of any of these crosses, it follows that this new character represents an allele of d, to which the symbol d^x is given.
Up to the present time over 25,000 seedlings of d/d^x hybrids have been grown without the appearance of a single d^+ seedling, thereby not yielding any evidence of unequal crossing-over.