204 NUTRITION 



by the plant : we shall note that the root will absorb the water 

 but the salts remain as a powder in the vessel, and soon the 

 plants cease to grow. If we get some powdered eosin, and 

 add it to the water, the eosin will dissolve and form a red 

 solution. Now, putting the uninjured roots of a plant to dip 

 in this solution, the roots become red externally and internally, 

 and the red colour will extend for some way up their interior. 

 Trying the same experiments with red particles of carmine, 

 the carmine does not dissolve, it remains suspended in the 

 water in the form of fine granules (like red mud), nor will any 

 red colour pass into the root. Thus the eosin passes into 

 the root because it is dissolved in water ; the carmine remains 

 outside the root because it is still solid and undissolved. A 

 flowering plant can absofd only liquids or gases : it cannot take 

 in solid bodies. 



Boots pour out an acid substance which, can dissolve some 

 minerals in the soil. — If the roots of a plant be made to grow 

 over and against a slab of marble, in time they make a pattern 

 of themselves on the surface of the marble. This is due to 

 the fact that the roots excrete a substance which corrodes 

 the marble by dissolving it wherever the young root touches 

 the surface. Placing a piece of blue Utmus-paper against the 

 absorbing part (see next paragraph) of a root, the paper 

 gradually assumes a red colour, thus proving that the substance 

 poured out by the root is of an acid nature. 



Only the terminal portions of roots absorb liquids. — The 

 region of the root a short distance behind the tip is the part 

 which absorbs. This region is usually marked out by the 

 possession of numerous root-hairs, which aid in the process 

 of absorption. Sometimes the roots do not possess root-hairs ; 

 this is often the case with plants grown by means of culture- 

 solutions, but still it is the same region of the root which takes 

 in liquid. Hence, if we cut off all the younger terminal parts 

 of roots the plant is liable to die. 



CONDITIONS INFLUENCING THE ABSORPTION OF 

 WATER BY ROOTS. 



(Use the apparatus given in fig. 241 for these experiments.) 



The Temperature of the soil, or of the culture-solution, affects 



the rate of absorption by roots. If we cool the culture-solution 



(by dropping pieces of ice into it or otherwise) the roots absorb 



