The plant whose seed husks provide soluble fiber used for constipation is psyllium.
Psyllium is the common name for several Plantago species whose seeds and especially their husks produce a gel-forming mucilage. The fiber passes largely undigested through the small intestine, absorbs water, and expands to soften and bulk the stool. That makes it a bulk-forming laxative and a widely used dietary-fiber supplement.
Commercial psyllium is cultivated mainly in northern India and is also known as ispaghula or isabgol. It is sold in powders, capsules, wafers, and fiber products. Unlike stimulant laxatives, it works by changing stool texture and supporting normal bowel movement rather than directly stimulating intestinal contractions.
Chia and flaxseed also contain fiber, but psyllium is the canonical answer because its seed husks are specifically standardized and widely used as a soluble-fiber constipation treatment. Psyllium must be taken with sufficient liquid; swallowing it dry can cause choking or, rarely, obstruction.