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Ultramafic outcrop near treeline
31726 VT, WTU
near plants of Polystichum lonchitis, P. lemmonii, P. kruckebergii
David Barrington (VT) writes 8 Jan 2024:
So, to get the crux of the matter your numbers 31726 and 31733 are confirmed as hybrids given on my study of the spores... 31726 also has irregular spores, but not many. So many of the sporangia remain unopened, though fully mature, and largely without contents except for what I Take to be a negligible amount of orphaned perispore clinging to the inside wall of the sporangium
These two do fall between the collections you sent of Polystichum lonchitis and P. kruckebergii morphologically. The character I found most useful was the ratio of the length to the maximum width for the medial pinnae—roughly 1:1 in P. lemmonii, 3:1 in P. lonchitis, and (though more variable) around 2:1 in P. kruckebergii. The two hybrids come in around 3:2.

Ultramafic outcrop near treeline, 31728 VT
David Barrington (VT) writes 8 Jan 2024: The character I found most useful was the ratio of the length to the maximum width for the medial pinnae—roughly 1:1 in P. lemmonii, 3:1 in P. lonchitis, and (though more variable) around 2:1 in P. kruckebergii. The two hybrids [31726, 31733] come in around 3:2.