This one gets a bit technical, folks, so buckle up.
(Makanu tane wa haenu; “Unsown seeds do not grow”)
“Nothing comes of nothing.” Things don’t happen without causes. You can’t expect to profit without doing any work.
蒔かぬ (makanu) is, in old grammar, a conjugated form of the verb 蒔く (maku), meaning to sow seeds, to plant, to sprinkle. It’s a verb of the 四段活用 (yodan katsuyou) type in imperfective form (changing maku to maka), and the nu is the negative zu suffix in attributive – in this case, prenominal – form. (“Prenominal” just means it comes right before a noun.) The noun it attaches to is 種 (tane, “seed”).
The second verb is actually 生ゆ (hayu), a 下二段活用 (shimo nidan katsuyou) type verb. These days, the yu form is lost and the “dictionary” form of the verb is haeru, a remnant of the imperfective and conjunctive forms of of the verb. In the classical grammar that the first verb obeys, though, we see hayu becoming hae, followed as before by zu in its attributive form to make ぬ (nu).
Something’s odd here, though. Why is the negative suffix in the attributive form? There are a few possible culprits. One is that the saying simply used to be longer, and used to contain a noun that has since been elided even while the prenominal verb form was preserved. Another is that the particle は (wa), or a structure that it was once a part of, changed the phrase’s ending from sentence-final to attributive form. Certain particles and situations would in fact cause this to happen, for some reason, although wa is not generally one of them. (Incidentally, wa here is probably acting in its emphatic or topic-marking role.)
Source is the Kyoto Iroha Karuta set. There are a number of related kotowaza, some synonymous (such as 打たぬ鐘は鳴らぬ, utanu kane wa naranu, “An unstruck bell doesn’t ring”) and some antonymous (such as 開いた口へ牡丹餅, hiraita kuchi e botamochi, “Adzuki-bean rice cake falling into an open mouth”). What I can’t find, however, is a longer form of the same kotowaza.
(“Sakka ni naritakattara, genkou wo tsukutte shuppansha ni renraku shinakereba narimasen yo. ‘Makanu tane wa haenu‘ desu yo.”)
[“If you want to become an author, you need to put together a manuscript and get in touch with a publisher. ‘Unsown seeds don’t grow,’ you know.”]