Question: In between Stratiotes and Clethra, which ones was described as a genus in 1753? 

Context paragraphs:
Astyanax (fish): Astyanax is a genus of freshwater fish in the family Characidae of the order Characiformes.  Some of these fish, like many of their relatives, are kept as aquarium pets and known collectively as tetras. With around 150 described species and new ones being described yearly, this genus is among the largest of the entire order; "Hyphessobrycon" also has more than 145 species and which one is larger at any one time depends on whether more species have been recently described in one or the other.  The blind and colorless cave tetra of Mexico is a famous member of the genus, but its taxonomic position is disputed: Some recognize it as part of the Mexican tetra ("A. mexicanus") and this is supported by phylogenetic evidence, but others recognize the cave form as a separate species, "A. jordani".

Stemonitis: Stemonitis is a distinctive genus of slime moulds found throughout the world (except Antarctica).  They are characterised by the tall brown sporangia, supported on slender stalks, which grow in clusters on rotting wood.  The genus was first described by German botanist Johann Gottlieb Gleditsch in 1753.  A 2014 estimate suggests that there are 18 species in the genus.  Identification within the genus is difficult, and can only be performed with confidence using a microscope.

Scaphyglottis: Scaphyglottis is a genus of orchids native to Mexico, Central America, northern South America and parts of the Caribbean.  The current concept of this genus is the result of combining several genera which have been described at various times.  The concept is characterized by the growth habit: not only are new pseudobulbs added at the base of the old ones (as is typical of sympodial orchids), but new pseudobulbs also grow at the apices of the old ones.  Many species are quite similar and difficult to distinguish, but some are clearly distinct.  A few have showy colors.  The genus comprises nearly 70 species.

Stratiotes: Stratiotes is a genus of submerged aquatic plant commonly known as water soldiers, described as a genus by Linnaeus in 1753.  Several specific names have been coined within the genus, but at present only one is recognized: "Stratiotes aloides".  native to Europe and NW Asia.

Iva (plant): Iva is a genus of wind-pollinated plants in the daisy family, described as a genus by Linnaeus in 1753.  Plants of this genus are known generally as marsh elders.  The genus is native to North America.

Theligonum: Theligonum is a genus of flowering plants in the Rubiaceae family.  It was described by Carl Linnaeus in 1753.  The genus is found from Macaronesia to the Mediterranean Basin, and from China to temperate eastern Asia.  It is the only genus in the tribe Theligoneae.

Myroxylon: The genus Myroxylon was originally described in 1753 by Linnaeus, such description was made using a specimen collected in the province of Cartagena (at the time Tolú was located in the province of Cartagena), and named it "Toluifera balsamum".  The genus "Myroxylon" was first established by Linnaeus filius in 1781, when he described "Myroxylon peruiferum" based on a specimen collected by Mutis in South America.

Boletus: Boletus is a genus of mushroom-producing fungi, comprising over 100 species.  The genus "Boletus" was originally broadly defined and described by Carl Linnaeus in 1753, essentially containing all fungi with pores.  Since then, other genera have been defined gradually, such as "Tylopilus" by Petter Adolf Karsten in 1881, and old names such as "Leccinum" have been resurrected or redefined.  Some mushrooms listed in older books as members of the genus have now been placed in separate genera.  These include such as "Boletus scaber", now "Leccinum scabrum", "Tylopilus felleus", "Chalciporus piperatus" and "Suillus luteus".  More recently, "Boletus" has been found to be massively polyphyletic, with only a small percentage of the over 300 species that have been assigned to "Boletus" actually belonging there and necessitating the description and resurrection of many more genera.

Purdiaea: Purdiaea is a genus of flowering plants in the family Clethraceae described as a genus in 1846.  It is one of two genera in this family, and was formerly classified in the related family Cyrillaceae.  "Purdiaea" is native to tropical regions of the Caribbean (with the highest species diversity on Cuba), Central America and northern South America, further south than the related genus "Clethra", the only other genus of this family.

Clethra: Clethra is a genus of flowering shrubs or small trees described as a genus by Linnaeus in 1753.

Supporting paragraphs: Stratiotes Clethra
Answer: Clethra