Dioicous, dioecious, monoicous and monoecious
In many bryophytes the antheridia and archegonia are produced on separate plants and such species are termed dioicous. As well as dioicous species there are also many monoicous species, where each plant carries both male and female organs. Within the monoicous bryophytes the antheridia and archegonia may be on the same or different branches or thallus lobes, depending on species.
There are two other words with very similar spellings: dioecious and monoecious. Many botanists would know these two. In a dioecious plant the male and female sexual organs are on separate plants, in monoecious ones you'll find both on the one plant.
Why are there pairs of terms, so similar in spelling and seemingly the same in meaning?
The story starts with Carl Linnaeus who divided the plants into 24 classes and two of those classes were named Monoecia and Dioecia. The latter part in each of those terms is derived from oikos, the Greek word for house. Taken literally the terms mean "one house" and "two houses" respectively. Linnaeus' classification scheme was based on how the male and female organs were arranged.
Of course Linnaeus said it all in Latin.
These Linnaean class names gave rise to the English words monoecious and dioecious, with the meanings noted above. You will also see spellings with the o and e joined as the ligature œ – hence:
The Oxford English Dictionary (OED) of 1971 notes that the words monoicous and dioicous were in use by the 1820s, as synonyms for the other two, possibly derived from the French words dioïque and monoïque, themselves derived directly from the Linnaean class names. Moreover, the OED also records the 19th century usage of the forms dioic and monoic, which are of course very similar to the French words in pronunciation. For interest sake, Webster's New Twentieth Century Dictionary of the English Language (2nd. edition, 1978) simply notes "same as dioecious" for dioic and dioicous and "same as monoecious" for monoic and monoicous.
The OED describes the forms dioic/dioicous and monoic/monoicous as rare or obsolete. In the English language there are numerous pairs of ostensibly synonymous words, but in many cases the different words in those pairs have different connotations. In such cases the different words will be kept in use for as long as the differences in connotation are important. Within the pairs monoecious/monoicous and dioecious/dioicous the words did not express different connotations to the biologists of the nineteenth and much of the twentieth centuries. It is therefore not too surprising to learn that only one form of each pair prevailed.
Given all that, why have the forms dioicous and monoicous re-appeared in the bryological literature? Bryophytes produce their sexual organs on haploid gametophytes whereas the flowering plants produce their sexual organs on diploid sporophytes. That being the case, various bryologists have argued that the terms for bryophytes should be different to those for the flowering plants. This has led to the more common use of dioicous and monoicous in bryological literature. However you will also find bryological sources (both print and web) which use dioecious and monoecious.
The terms dioecious and monoecious are also used in zoology: monoecious meaning "two sexes in one individual" and dioecious meaning "two sexes in separate individuals". Having said that, it is important to note that hermaphrodite is now the more common zoological term for a monoecious individual.
Variations of monoicous
In a monoicous bryophyte the female and male sex organs may be positioned in various ways on the one plant.
When talking of leafy bryophytes the term inflorescence is used to denote a cluster of sex organs along with any modified surrounding leaves. In an autoicous bryophyte the male and female organs are in separate inflorescences. If the male and female organs are in the same inflorescence there are two possibilities. If the sexes are clustered separately in the one inflorescence, the bryophyte is described as paroicous. If the sexes are not clustered separately the bryophyte is synoicous or gynandrous. Within the autoicous leafy bryophytes you can have even more complexity but, for the purpose of this website, there's no need to go into the variety of inflorescence arrangements in autoicous bryophytes.
The monoicous thallose bryophytes also show a variety of spatial arrangements of the sex organs. The term inflorescence is clearly not applicable to thallose bryophytes but there are terms to describe those spatial arrangements in thallose bryophytes. Once again, for the purpose of this website, there's no need to go further into that topic.
It is intriguing that while the spellings dioicous and monoicous are used in order to stress the differences between the haploid-dominant bryophytes and those plants where the diploid stage is dominant, everyone seems happy to talk of inflorescences in the leafy bryophytes. The core of the word inflorescence is derived from the Latin word for flower. At one stage the bryophytes were thought to possess flowers. The thinking was that all plants have flowers, bryophytes are plants and so they must have flowers. Of course bryophytes do not have flowers but the same word inflorescence is still used in connection with both haploid-dominant bryophytes and diploid-dominant flowering plants, albeit now with different meanings.
There are differences of view amongst botanists, some holding that terms should have very narrow definitions whereas others would allow more latitude. Hence there are those who would hold that there should be bryophyte-specific terms for all structural features, while others don't not hold that view. The result is that in bryology there both bryophyte-specific terms as well as general plant terms. You've already seen inflorescence as one example of the latter. Other examples are leaf, nerve, perianth and calyptra. Perianth and calyptra have no rival, bryophyte-specific terms. Various bryologists have argued for the use of the term phyllidium, rather than leaf, but that alternative is not in wide use. By contrast costa, an alternative term for nerve, is in very widespread use – especially in the technical literature. You are more likely to encounter the word nerve in semi-popular bryophyte literature.