242 THE CULTURE OF FARM CROPS. 



can be easily studied. The root hairs will be found attached, 

 much like the bristles to a bottle brush, to the -smallest fi- 

 bers of the root. These so-called root hairs are the absorp- 

 tive organs of the roots. As the plant grows, the roots 

 gradually become stronger harder and more woody, as is 

 consistent with their mechanical purpose to uphold the 

 plant; and the latest growth of roots only, become feeders. 

 The precise way in which roots absorb the plant food, 

 and their functions are accomplished, is still a matter of 

 some uncertainty; but the process of osmose is probably 

 that by which the solutions of the various substances of 

 which plant food consists are absorbed. The dissolved mat- 

 ter in the soil in which the roots are immersed, passes 

 through the cell membranes in the manner previously de- 

 scribed, together with as much water as may be needed to 

 supply the needs of the plants. It is common to say that 

 the food enters through the pores of the roots; but this is 

 more a figure of speech than a reality; for under the pres- 

 ent belief in regard to the action of osmose the existence of 

 pores or openings in the membranes is not necessary. How- 

 ever it is very well ascertained that no solid substance, how- 

 ever finely divided it may be, can enter the roots of plants; 

 and only such as is dissolved in w T ater. 



Nor do the roots absorb air or other gaseous matter un- 

 less it is dissolved in water. If a plant is grown in water 

 in which carbonic acid gas is dissolved, the gas gradually 

 disappears as it is extracted by the roots. If a plant is 

 grown with the roots in a bottle partly filled with water, the 

 air in the bottle is gradually deprived of its oxygen to re- 

 place that which has been extracted from the water. But 

 if instead of air, the bottle is filled up with carbonic acid, 

 the plant will droop and soon die; the same will happen if 

 nitrogen, or hydrogen gas, is substituted for atmospheric 

 air. This should not be accepted as a proof that these gases 

 are noxious to plants, but rather that they exclude the oxy- 

 gen which is indispensable for all living beings; plants or 

 animals. 



That roots select their food from a variety of substances 



