The second declension, like the other declensions, has a diagnostic ending in the genitive singular. That ending was -ae in the first declension, and it is -ī (a long i) here in the second one.

The second declension is a little more varied than the first. Almost all first declension nouns are feminine, but all three genders are represented in the second declension. First declension nouns end in -a, but second declension nouns have a nominative singular ending of -us (mostly masculine, some feminine; Tables 3.1 and 3.2), -er (mostly masculine; Tables 3.3 and 3.4), -ir (only one, masculine; described, no table), or -um (neuter; Table 3.5).

-us nouns. To form the second declension of -us nouns, drop the -ī from the end of the genitive singular—this gives you the stem—and add the endings shown in Table 3.1 below.

Most second declension nouns that end in -us in the nominative singular are masculine. Notable exceptions are many trees and shrubs (e.g., Alnus, Cedrus, and Cornus), which are feminine. Other exceptions are the feminine nouns humus (ground), hydathodus (hydathode), and methodus (method), and the neuter nouns pelagus (sea) and virus (poison).

-ir noun. The only -ir noun is the masculine vir (man), but this is not found in Linnaeus’s Species Plantarum. It has the same endings as the other nouns described above, except in the nominative singular, which is simply vir.

Latin Descriptions and Diagnoses
In lesson 2 you saw the phrase “Latin descriptions and diagnoses” used several times. It seems appropriate, then, to take some time here to explain what a description is and what a diagnosis is. A description is simply a listing of the features of a plant. It can be as simple as planta herba (plant an herb) or it can go on for pages. But whatever their length, they all follow the same pattern: a noun followed by words or phrases that describe that noun, those words or phrases separated by commas, then a semicolon, then another noun followed by descriptors that are separated by commas, and so on. A description might go something like this: plant an annual herb, armed with prickles, synoecious; leaves opposite, pubescent on the abaxial surface, glabrous adaxially; flowers red, with 5 petals, 4 sepals, and 37 stamens; fruit a pink berry, 3–24 mm in diameter, densely covered with bristles. In a description, you essentially start with nothing and slowly build an image of the plant.
   A diagnosis, on the other hand, always begins with a known taxon and proceeds to note how a new taxon of the same rank differs from the known one. An example of a diagnosis for a new species could be: similar to Lentibularum officinale but it [the new species] differs with blue flowers and three fused carpels.