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TEXTBOOK OF PLANT PHYSIOLOGY 



particles. It is almost impossible to wash these away without 

 injuring the tissues. (Fig. 31.) 



Such a close contact of the root hairs with soil particles has 

 still another significance. The roots can utilize not only solutes 

 and adsorbed substances, but they are able to dissolve substances 

 that are insoluble or little soluble in water. This function of the 

 roots was discovered by Sachs in a very simple experiment, which 

 has since been adapted for school demonstration. A carefully 

 polished marble plate is inserted horizontally or obliquely in the 



Fig. 30. — Attachment of root hairs 

 to soil particles (after Molisch). 



Fig. 31. — A radish seedling with 

 a soil coating around the roots 



(after Molisch). 



soil in which plants are grown, legumes being best suited for this 

 purpose. When the roots have developed, and some of them have 

 come in close contact with the marble plate, it is taken out and 

 washed. On the polished surface of the plate will now be seen 

 distinctly small traces left by the roots (Fig. 32). 



It is possible to observe in the same way the dissolving effect 

 of the roots upon natural rock phosphates. Sometimes in water- 

 culture solutions, nutrient elements are introduced in the form of 

 solid substances, which are gradually used by the plants during 

 their growth, as in Crone's solution, where phosphorus is supplied 

 in the form of water-insoluble Ca 3 (P0 4 )2 and Fe 3 (P0 4 )2. 



The mechanism of the dissolving action of roots is not under- 

 stood. An important function is performed by CO2 excreted by 



