460 



Introduction to the Study of Science 



in their growth and maturity. Other substances contained 

 in soil water and sometimes found in the ash of plants are 

 sodium, chlorine, fluorine, and sulfur. Besides these soil water 

 provides plants with hydrogen, oxygen, and some carbon. What 

 amounts of these substances plants derive from soil water 

 may be seen in the table, page 464. How indispensable they 

 are to plants and to all living organisms may be inferred 

 from the fact that they are all constituents of the protoplasm, 

 the living substance, of the cells of which an organism is 

 composed. 



222. How plants obtain soil water. Plants utilize enormous 

 quantities of soil water, and more than half of the total weight 

 of a living plant is due to its water content. 

 Water is absorbed through the delicate 

 root hairs and possibly through the tender 

 areas of the roots adjacent to the root hairs. 

 The sap of plants is a concentrated solu- 

 tion of many different substances, such as 

 sugars and salts. This solution is denser 

 than soil water, which, as before stated, 

 is a weak solution of the substances dis- 

 solved from the soil. In the following 

 exercise the plant sap may be represented 

 by a sugar sirup or molasses, and the soil 

 water by ordinary water. To represent 

 the membrane covering the root hairs a 

 piece of bladder or of sheet rubber may 

 be used. This membrane has no openings, 

 but is permeable by water. The apparatus may be arranged 

 as in Fig. 146. 



Exercise. Cover the large end of a long-stemmed thistle tube, a 

 funnel, or a tall, narrow lamp chimney with a piece of bladder or sheet 

 rubber to be stretched tight and tied securely with a waxed string. 

 Fill the bulb and one to two inches of the stem with the molasses or 

 sugar sirup and arrange, as in Fig. 146, in a wide-mouth bottle or pan 



FIG. 146. Arrange- 

 ment of apparatus to 

 show osmosis. 



