What Johnson meant was that with his shortsightedness he'd have to creep along the ground on all fours to be able to see those things that it is necessary for a botanist to see.

约翰逊的意思是这样：以他的近视程度，他不得不四肢着地的趴在地上才可以看到那些对于植物学家来说必须要看到的东西。