for the most part, close up, flower stem
梗 centers on a central, obstructive, or stem-like core: a flower stem that blocks or closes up, and the idea of something being mostly or essentially that core. The meanings "for the most part" and "close up" extend from this central, blocking stem.
梗 combines 木 (tree/wood) with 更, which likely contributes a sound element. The original meaning relates to a thorny or obstructive plant stem, which later extended to blocking and the idea of a main stem or core.
A tree (木) with a stiff, obstructive stem (更) that blocks the way. This stem is the main part, so it represents the core or "for the most part" idea, and it can close up a path like a flower stem blocking a passage.
For コウ, imagine a core (コウ) stem that is hard and obstructive, like the core of a tree blocking your path. The word "core" sounds like コウ and ties to the central stem meaning.
stoppage; blocking
heart attack; myocardial infarction
Chinese bellflower (Platycodon grandiflorus)
bok choy (Brassica rapa subsp. chinensis; esp. cultivars with green stalks); pak choi
outline; summary; abstract
lacunar infarction
hairyflower bellflower (Campanula chamissonis)
pedicel
mountain harebell (Campanula lasiocarpa); Alaska harebell
acute myocardial infarction; AMI
twisted Chinese bellflower crest
Lobelia sessilifolia (species of lobelia)
spinal cord infarction
cerebral infarction; (ischemic) stroke
flower stalk; peduncle