a hollow fiber formed of hydrophilic material and having permeable walls is treated to enhance the permeability . reactions used to treat the fiber have a gas inside and gas outside , a gas inside and liquid outside , a liquid inside and gas outside , and a liquid inside and a liquid outside . the treatments generally involve the plasticizing of the fiber material while having the inside of the fiber at a higher pressure than the outside . these treatments produce a structural change in the fiber from a flexible tube having strong walls to expanded , rigid and somewhat fragile walls . where more than a short length of the treated fiber is to be used , an open mesh cage is used to confine the fiber to a desired volume during treatment and to protect it subsequently . a technique for joining flexible tubing to the treated fiber is presented . the treated fiber may be used in apparatus and methods involving osmosis and reverse osmosis . novel plant irrigation methods and apparatus are disclosed , and structures for changing the solvent quanties of solutes and measuring the osmotic pressures of solutions are also presented .