ABSORPTION AND OSMOSIS 61 



Hydrotropism. Roots respond similarly to the presence of 

 water, turning toward moisture even at long distances. This 

 tendency, called hydrotropism, is very useful, especially if soil 

 water be scant. Vast numbers of fine roots are often found project- 

 ing into springs and streams, forcing their way into water pipes 

 or piercing deep into the soil, led by this force that turns them 

 toward the needed moisture. 



Selective Absorption. Another fact connected with absorption 

 is, that plants, though growing side by side, take very different 

 matters from the same soil. This apparent impossibility is ac- 

 complished by the action of the protoplasm which lines the inner 

 walls of all active cells and has the remarkable power to select, 

 in a considerable degree, what substance the roots shall absorb 

 with the water. This selective absorption, as it is called, accounts 

 for the variety of food substances taken from the same soil by 

 different plants. 



Successive Osmosis. All this arrangement for absorption would 

 be useless, were there not some way provided for passing on the 

 absorbed liquids after being taken up by the root hairs. When 

 the outer layer of cells has taken in soil water their contents are 

 diluted, and they become less dense than those next within. Their 

 contents tend to pass to the next inner layer, as the osmotic 

 current is always toward the denser liquid. 



This last step removes the newly absorbed soil water from the 

 epidermal cells and leaves them denser again, ready to absorb 

 more soil water from without. 



Root Pressure. This process continues inward, from cell to 

 cell, till the ducts are reached, when the liquids rise up through 

 root and stem, causing the uplift which is known as joot pressure. 



This root pressure is one important cause of the circulation of 

 sap in plants, and is often sufficient to raise the water to heights 

 of one hundred feet or more. But neither this nor any other known 

 cause is equal to the task of lifting water as high as some of our 

 tallest trees, and the method by which that is done is still unknown. 

 This inward osmosis may be reversed by putting salt in the soil. It 

 dissolves in the soil water, makes it denser than the contents of 



