Hawkweed teaching unexpected lessons

My dear Nägeli,

The packet of Pisum seed went out to you on the third of this month, thirty varieties by number, and I write now not about the peas but about the hawkweed, because the hawkweed is teaching me something I did not expect to learn. You will recall my trouble with Hieracium. In the peas, the hybrids obey. Cross smooth with wrinkled and the F1 is all smooth; let it self and the F2 splits, 5474 round against 1850 wrinkled in the last full count, close enough to three to one that the deviation sits within what chance allows. The factors are discrete. They do not blend. I could count my way to the mechanism.

The hawkweed will not count. The F1 plants from the same cross, grown side by side in the same bed at the monastery garden, come out unlike one another, and their seed does not segregate in any ratio I can find. For a season I suspected my method. Then I read what Nägeli - you - had written on apogamy in these very plants, and the suspicion moved from my hands to the plant itself. The hawkweed may be forming seed without fertilization at all. If so, the hybrid is not a hybrid in the sense my peas were, and no law of segregation should be expected to appear, because the machinery the law describes is simply not engaged.

Here is the thought I want to put plainly before you. The pea result sat in the archive for years looking like an oddity about garden vegetables, and the hawkweed result now sits in front of me looking like a failure of the law. Both appearances may be artifacts of the instrument. In the pea case the instrument was patience, ten thousand seeds, counts large enough that the ratio could not hide. In the hawkweed case the missing instrument may be the microscope - the embryo sac, the development of the egg without pollen. The observation was available all along; what was absent was the means of seeing the cell that carries the exception.

I have begun sectioning the ovules this week, thin cuts of the young ovary, stained. The first slides show development proceeding in the absence of the pollen tube, though I will not say more until I have forty sections rather than six.

By spring I will know whether the hawkweed breaks the law or merely bypasses it, and I will write you first.

Your servant, G. Mendel