70 ROOT AND CROWN. 



The guiding of the water to the rootlets is of 

 the greatest importance, for the water is not only 

 absorbed by them, but is carried, as we shall see 

 later, over the whole plant. . . . 



In the building up of the molecules of sugar, of 

 starch, of cellulose, and of all important substances 

 out of which the plant is formed, the atoms of 

 water are used as building stones, and without 

 water no growth could take place. From this 

 point of view water must be regarded as an indis- 

 pensable foodstuff of plants, no less than the car- 

 bonic acid of the air. Water, however, plays 

 another weighty part in the life of the plant. The 

 mineral salts which nourish water-plants, earth- 

 plants, and air-plants, as well as the organized food 

 on which parasitic plants feed, can be taken up 

 only in solutions. The salts can pass through the 

 cells only when their walls are saturated with 

 water, and must be dissolved in water in order to 

 be brought into the interior of the plant wherever 

 they are needed. Water acting in this capacity in 

 living plants is to be regarded as the motive power. 

 As the mill by the brook works only when its 

 wheels are put in motion by the water, so the 

 living, growing plant demands a great quantity of 

 available water in order that its complicated life 



