"as se'irim upon the herbage and as showers": A man who starts learning Torah does not know how to proceed until he has learned two orders or two books, after which it "pursues" him, "as showers on the grass."
Variantly: "My taking shall drip as the rain": Just as rain is one, and it descends on the trees and imparts to each a distinct flavor: to the grapevine, in accordance with its nature, to the olive tree, in accordance with its nature; and to the fig tree, in accordance with its nature — so words of Torah are all one, and they "impart the flavors" of Scripture, Mishnah, halachoth, and aggadoth.