1 82 OSMOSIS : ABSORPTION OF WATER 



Firmly tie the rubber tube to the glass tube and to the stem of the plant, 

 and then partially fill the glass lube with water taking care that no air is left 

 between the end of the stem and the water. Pour in mercury until the level 

 in the free limb of the tube is considerably higher than in the other [6) ; the 

 pressure of the mercury will force the water (a) into the shoot and the leaves 

 will soon begin to assume their natural position and firmness. 



2. Absorption of water. — In all acdvely-growing plants water 

 forms considerably more than half their total weight; it satur- 

 ates the living protoplasm and the cell-walls, and is the chief 

 component of the cell-sap. 



Water is utilised by plants for maintaining the turgidity 

 of their cells, and a small amount is employed as a food-material. 

 It is also of the greatest importance for the purpose of dissolving 

 the various foods present in the plant and conveying them to the 

 different organs requiring nourishment. Moreover, the absorp- 

 tion of water is the only means which a plant possesses of 

 obtaining the various essential food-materials which are derived 

 from the soil, for it is only when these necessary constituents 

 are dissolved that they can find an entrance into plants : no 

 solid particles of manures or other components of the soil, how- 

 ever small, are taken up by them. 



Water and the dissolved compounds which plants absorb pass 

 into them by osmosis and therefore only gain an entrance 

 through organs whose external cell-walls are uncutinized or 

 unsuberized. During the life of an ordinary farm or garden 

 plant, the absorption of water and the absorption of dissolved 

 food-materials are necessarily carried on at the same time : they 

 may, however, be treated as separate phenomena. 



The nature of the dissolved substances which are absorbed 

 by plants, and the conditions which govern their absorption, are 

 dealt with in chapters xii. and xv. ; at present it is advisable to 

 consider the absorption of water alone. 



Plants which live completely immersed in the sea and in 

 ponds and rivers rarely have a well-developed cuticle and take 



