A.T. Robertson Commentary Mark 4:27

"and should sleep and rise night and day, and the seed should spring up and grow, he knoweth not how." — Mark 4:27 (ASV)

Should sleep and rise (καθευδη κα εγειρητα). Present subjunctive for continued action. So also

spring up and grow (βλαστα κα μηκυνητα) two late verbs. The process of growth goes on all night and all day (νυκτα κα ημεραν, accusative of time).

He knoweth not how (ως ουκ οιδεν αυτος). Note position of ως (beginning) and αυτος (end) of clause:

How knows not he . The mystery of growth still puzzles farmers and scientists of today with all our modern knowledge. But nature's secret processes do not fail to operate because we are ignorant. This secret and mysterious growth of the kingdom in the heart and life is the point of this beautiful parable given only by Mark. "When man has done his part, the actual process of growth is beyond his reach or comprehension" (Swete).