Montsechia is an extinct genus of aquatic plants containing the species Montsechia vidalii, discovered in Spain.
 "Montsechia vidalii" lived about 130 million years ago, during the Barremian age, and appears to be the earliest known flowering plant.
 It has affinities with the modern genus "Ceratophyllum".
Closterovirus, also known as beet yellows viral group, is a genus of viruses, in the family Closteroviridae.
 Plants serve as natural hosts.
 There are currently 11 species in this genus.
 Diseases associated with this genus include: yellowing and necrosis, particularly affecting the phloem.
 This genus has a probably worldwide distribution and includes among other viral species the "Beet yellows virus" (the type species) and "Citrus tristeza virus", rather economically important plant diseases.
 At least some species require vectors such as aphids or mealybugs for their transmission from plant to plant.
Sarcodes is a monotypic genus of a north-west American, springtime, flowering plant in the heath family (Ericaceae), containing the single species Sarcodes sanguinea, commonly called the snow plant or snow flower.
 It is a parasitic plant that derives sustenance and nutrients from mycorrhizal fungi that attach to roots of trees.
 Lacking chlorophyll, it is unable to photosynthesize.
 Ectomycorrhizal (EM) symbioses involve a mutualism between a plant root and a fungus; the plant provides fixed carbon to the fungus and in return, the fungus provides mineral nutrients, water and protection from pathogens to the plant.
 The snow plant takes advantage of this mutualism by tapping into the network and stealing sugars from the photosynthetic partner by way of the fungus.
 This form of parasitism is known as mycoheterotrophy.
Redheadia is a genus of fungi in the family Sclerotiniaceae.
 It is a monotypic genus, containing the single species Redheadia quercus, described in 2005 by Suto and Suyama.
 "R. quercus" is the teleomorph form of the fungus "Mycopappus quercus", which causes a plant disease called "frosty mildew" in the Sawtooth Oak ("Quercus acutissima").
 The genus is named after Canadian mycologist Scott A. Redhead who first described the anamorph genus "Mycopappus", while the specific epithet is named after the genus of the host plant.
Ceratophyllum echinatum, commonly called spineless hornwort, is an aquatic perennial plant of the genus Ceratophyllum.
 It can be found in ponds and lakes.
 It is principally an eastern North American species and the only species of its genus endemic to North America.
Phylloglossum, a genus in the clubmoss family Lycopodiaceae, is a small plant superficially resembling a tiny grass plant, growing with a rosette of slender leaves 2-5 cm long from an underground bulb-like root.
 It has a single central stem up to 5 cm tall bearing a spore-producing cone at the apex, and was previously classified variously in the family Lycopodiaceae or in its own family the Phylloglossaceae, but recent genetic evidence demonstrates it is most closely related to the genus "Huperzia" and is a sister clade to the genus "Phlegmariurus", which was formerly included in "Huperzia".
Lophodermium is a genus of fungi within the family Rhytismataceae.
 The genus contains 145 species and has a global distribution.
 Species of this genus are usually observed producing zone lines, conidiomata and ascomata on dead fallen leaves, but at least some are known to colonize living leaves.
 In many cases they then live inside the colonized leaf as a symptomless endobiont, where they are regarded as detritivores utilising dead plant matter.
 In a few cases they may kill all or part of the leaf prematurely, and there is a substantial literature dealing with those species as plant pathogens.
 The genus infects many different plant families but with a notable concentration in the family Pinaceae; many "Lophodermium" species are restricted to a single host genus (or even species), but some, particularly those infecting grasses, may infect several genera.
 Some are economically important plant pathogens, such as those that cause needlecast disease in European Black Pine, Scots Pine and Red Pine in forestry and christmas tree plantations.
 In these species, notably "L. pinastri" and "L. seditiosum", the fungal spores disperse and infect the pine needles in late summer, which turn brown by the following spring and then fall off.
Conoclinium, the mistflowers, is a genus of four species of herbaceous perennial flowering plants, native to North America.
 They are 0.5 to tall, and have blue to purple or violet flowers (occasionally white).
Conoclinium coelestinum, the blue mistflower, is a North American species of herbaceous perennial flowering plant in the sunflower family.
 It was formerly classified in the genus "Eupatorium", but phylogenetic analyses in the late 20th century research indicated that that genus should be split, and the species was reclassified in "Conoclinium".
Ceratophyllum is a cosmopolitan genus of flowering plants including four accepted species in 2016, commonly found in ponds, marshes, and quiet streams in tropical and in temperate regions.
 It is the only genus in the family Ceratophyllaceae, itself the only family in the order Ceratophyllales.
 They are usually called coontails or hornworts, although hornwort is also used for unrelated plants of the division Anthocerotophyta.

Question: Which plant is the only genus in the family, Ceratophyllum or Conoclinium?
Answer: Ceratophyllum