Since September 1972, Gea Zijlstra is working for the Index Nominum Genericorum (ING). This is a compilation of generic names published for organisms covered by the ICN: International Code of Nomenclature for Algae, Fungi, and Plants.

The ING is a collaborative project of the International Association for Plant Taxonomy (IAPT) and the Smithsonian Institution in Washington (DC), USA. ING was initiated in 1954 as a compilation of generic names published for all organisms covered by the International Code of Botanical Nomenclature.

The original intent of ING was to bring all generic names of plants together in a single list to reveal homonymy between groups. In addition, ING includes bibliographic citations and information about the typification and nomenclatural status of generic names.

The index was first published in 35 sets of each 1000 printed cards during the  Oct. 1, 1955 – Nov. 1, 1971 period.

In 1979, ING was published in book form in three volumes, then including close on 63.500 records [Farr, Leussink & Stafleu, Index Nominum Genericorum (Plantarum) in IAPT’s book series (Regnum Veg. 100-102)]. These books also included the 35.000 records that previously had been published in cards (with a few of them already in a corrected format). A first Supplement (including additions and corrections) was published in 1986 (Farr, Leussink & Zijlstra, Regnum Veg. 113).

While a second supplement was planned, it was realized that hard copy would be immediately out of date. So it was decided to give this work a place on the World Wide Web: http://botany.si.edu/ing/, this happened on February 6, 1996. Every month Ellen Farr updates the working version from a computer at the Smithsonian to the web version. A total of 116 taxonomists from all over the world have contributed to ING. From the beginning, it has been ING’s policy to verify all names in the original literature. Where this could not been done because it was not available, the phrase 'non vidi' follows the reference for the name.

From the beginning, current work on ING is supported by the Smithsonian Institution, IAPT, and the University of Utrecht, The Netherlands. The Utrecht commitment with ING started when the IAPT (founded by Prof. Dr. J. Lanjouw) still was housed at the Utrecht Institute of Systematic Botany.

Since 2008, Gea tries to confine herself to names of fossil plants, because these are not indexed anywhere else; whereas for names of recent plants, IPNI, Tropicos, the Index Nominum Algarum and the Index Fungorum are available as well. When for a certain generic name, one enters a search in ING, it presents the data from those indices as well.

In her checking of literature, to find generic names that should be added to ING, Gea regularly comes across names that are in use, but should not be used, e.g. because for this genus an earlier name is available; or it should not be used because the same name was created earlier for another genus (a homonym – the original reason to start with ING). Then the International Code of Nomenclature for Algae, Fungi, and Plants (ICN) (McNeill & al., Regnum Veg. 154. 2012) offers a solution: it is possible to propose such a name for conservation. It is always easy to find a colleague who is familiar with the taxonomy of the group concerned, as a co-author to write a proposal. Besides this, Gea is regularly engaged in discussions on interpretation of the rules from the Code, and/or a question if some name might be worth conservation – this also now and then gives rise to a proposal to conserve a name.