Abortion of Spores and Sporangia as a Tool in the Detection of Dryopteris Hybrids

Abstract

It is practically axiomatic that interspecific and intergeneric in ferns are, with rare exceptions, sterile. The situation found by Trevor Walker (1958) in certain of Pteris remains our only example of interspecific hybridization that yields fertile progeny directly. Because sterility is otherwise so nearly universal among fern hybrids, it seems desirable to describe some of the signposts of this condition. What is meant by sterile requires explanation, because there have been some misunderstandings. The mere absence of sori, for example, is not necessarily diagnostic for hybridity. So far as has been determined, fern have just as many sori as do normal species. Species as well as both tend to lack sori when growing, under conditions unusual for the plants involved, such as exceptionally deep shade and especially wet, rich substratum. The point is that production of sori and of sporangia per se are not inhibited in interspecific crosses, so that a hybrid plant which is unable to reproduce itself may nevertheless be abundantly soriferous. A more serious problem, as will be exemplified by alleged hybrids described and figured by Crane (1953) and Tryon (1942) to be discussed below, involves misinterpretation of the criteria for the detection of hybrids. We hope, therefore, that the following notes and illustrations will prove useful to those who are engaged in clarifying the variations and taxonomy of American woodferns, Dryopteris.