Recall that all declensions have a diagnostic ending in the genitive singular. You have already learned that the genitive singular ending of all first declension nouns is -ae and that the genitive singular ending of all second declension nouns is -ī. In the third declension, the diagnostic genitive singular ending is -is.

You have seen an increase from only one nominative singular ending in the first declension to four endings (-us, -er, -ir, -um) in the second declension and from mainly one gender (feminine) in the first declension to mainly two genders (masculine and neuter) in the second. In the third declension there are many nominative singular endings, and there are nouns of all three genders. There are also a variety of letter changes between the nominative and genitive singular in the third declension instead of just the occasional dropping of an e in -er nouns of the second declension. Some books present many rules (with exceptions) to help you determine what these changes are. The approach taken here is to recommend that you simply learn both the nominative and the genitive singular of each noun, which you should do anyway in order to know which declension a noun belongs to.

Tables 5.1 and 5.2 introduce the third declension endings for masculine and feminine nouns. Note that these genders have the same endings as each other. Also note that the nominative singular is not truly a “stem plus an ending”, so it is referred to in the table as the “base word”.

Tables 5.3 and 5.4 introduce the endings for neuter nouns of the third declension. Note that the nominative and accusative singular are the same as each other and that the nominative and accusative plural are the same as each other, the same pattern observed in neuter nouns of the second declension.

There are some third declension nouns, called I-stem nouns, whose endings differ slightly from the endings above. Table 5.5 summarizes the characteristics of these nouns and how their endings differ from those of regular (i.e., non–I-stem) third declension nouns.

There are, as you probably expected, exceptions to the above. However, you will probably encounter them very rarely in botanical Latin descriptions and diagnoses.

Vocabulary (* = I-stem nouns). For more third declension nouns, pleae visit the online Botanical Latin Glossary and search for "3rd declension" (without the quotation marks).

apex, apicis (m.) – apex	pars, paris (n.) – pair
arbor, arboris (f.) – tree	* pars, partis (f.) – part
* caulis, caulis (m.) – stem	pollen, pollinis (n.) – pollen
flos, floris (m.) – flower	radix, radicis (f.) – root
genus, generis (n.) – genus	semen, seminis (n.) – seed
margo, marginis (m. or f.{?}) – margin	stamen, staminis (n.) – stamen
* nectar, nectaris (n.) – nectar	teges, tegetis (f.) – mat

Latin Descriptions and Diagnoses
{Give example(s) of a group A adjective modifying a third declension noun.}

Exercises
Translate the following into English.
   1. caulis glaber
   2. arbores parvae
   3. radices herbae
   4. inflorescentiae florum
   5. genera plantarum
   6. stamines pollinibus
   7.
   8. radicibus crassis
   9.
   10.

Write the following in Latin.
   1.
   2.
   3.
   4.
   5.
   6. inflorescence of white flowers
   7.
   8.
   9.
   10.

Write a Latin description for the following. These are more challenging. You have to convey the meaning rather than following the exact wording.
   1.
   2.