non-glutinous long grain rice
秈 refers specifically to a type of rice: non-glutinous, long-grain rice, distinct from the stickier short-grain varieties.
秈 combines 禾 (grain/rice plant) with 山 (mountain), likely indicating a type of rice grown in mountainous or upland areas, distinguishing it from paddy rice.
The grain radical 禾 beside a mountain 山 suggests rice growing on mountain terraces—this is the long-grain, non-sticky rice that thrives in such conditions.
For セン, picture a thousand (千, せん) grains of this long rice lined up on a mountain path: 千 -> セン, and each grain is a 秈.