Rose (Rosa rugosa Thunberg) achenes are water permeable. This was shown by the weight increases after immersion into a phosphate buffer. The imbibition curves were typical but, contrary to most imbibing seeds that swell, rose achenes do not increase in size. The water uptake of seeds of intact achenes is impeded. A comparison of intact achenes with achenes that were cut in half showed that the weight increase of cut achenes after immersion in the buffer was significantly higher (P < 0.01) and also that the penetration of stain into the embryos of cut achenes was significantly increased (P < 0.01) after immersion into a 1% eosin solution. The restricted water penetration into seeds of intact achenes is believed to be due to a reduction of the imbibition pressure, caused by an increased wall pressure.