WORK OF THE ROOT 49 



root-hairs. We will determine the important function of 

 the root-hair region with the help of a few experiments. 



Acids excreted by roots. Obtain a piece of polished marble, 

 place it in the bottom of a shallow box, and cover it two 

 to three inches deep with fine wet sand. In this, place 

 a few seeds of Sunflower, allow them to germinate, and 

 in ten to twelve days examine the seedlings, noting how 

 the roots, unable to descend vertically, have spread over 

 the marble slab. Now examine the surface of the slab and 

 see the change that has taken place wherever the roots of 

 the seedlings have touched it. The tracks of the roots are 

 clearly marked by the corrosion of the polished surface. 

 Now take a little dilute hydrochloric acid, dip into it 

 a small camel-hair brush, and write the date of the experi- 

 ment on the polished surface. When this is dry it will be 

 seen that the course taken by the brush is etched on the 

 surface, just as was the course taken by the roots. A small 

 piece of cotton thread dipped in the acid and laid on the 

 slab will similarly leave its trail. Can it be that the young 

 roots and their root-hairs during their active growth excrete 

 a substance capable of etching marble ? They do, for we 

 have already seen (p. 44) that germinating seeds give off 

 much carbon dioxide, and this gas in the presence of water 

 is able to dissolve marble. It is probable that young roots 

 are able to excrete other acids capable of dissolving mineral 

 substances in the soil. Thus they may bring into solution 

 substances otherwise difficult to dissolve, and these may 

 be absorbed as food by the plant. 



Wilting. Allow a plant growing in a flower-pot to 

 remain un watered for a few days, and note what happens. 

 Now water the soil and notice what changes take place. 

 Obtain a young, leafy shoot of Laburnum and notice how 

 soon the leaves droop. Place the cut end in water, cut 

 off a small piece (under water), and note how quickly the 

 shoot revives. We see that, as the soil loses water, the 



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