THE ORGANIZATION OF THE PLANT 63 



Osmosis. Root hairs absorb from the soil by a physical 

 process known as osmosis. In this, when two liquids of different 

 densities are separated by a membrane, such as a cell wall and 

 its lining of protoplasm, there is at once set up a tendency for 

 each liquid to pass through the membrane to the other until 

 both liquids are of equal density. In osmosis the current from 

 the less dense liquid into the denser is always the stronger. 

 One can illustrate the process very well by filling a small jar 

 with molasses and tying a piece of parchment paper or hog's 

 bladder over the open end to represent a cell, and immersing 

 this jar in a larger jar of clear water for a few hours. If care 

 has been taken to make a water-tight joint between the jar and 

 its parchment cover, sufficient clear water will pass through 

 the membrane into the molasses to distend the covering of the 

 jar to its utmost. In the plant the evaporation of water from 

 the leaves or its use in forming plant food renders the sap in 

 the cells more dense than the soil water, and this consequently 

 flows into the plant, carrying the dissolved minerals with it as 

 these processes continue. Once in the root, the water spreads 

 from cell to cell through the cortex until it finally reaches the 

 ducts in the central cylinder and is sent upward to the shoot. 

 The root hairs not only absorb the water that clings to the 

 soil particles with which they come in contact, but this absorp- 

 tion sets up a capillary movement which drains adjacent parti- 

 cles of their moisture. If there should happen to be a large 

 enough amount of any soluble substance in the soil, the cur- 

 rent of water would set outward from the plant and cause its 

 death. This is what happens when we put salt upon weeds 

 or grass, and explains why ordinary plants cannot live in 

 alkali soils. 



The stem. For convenience the shoot may be divided into 

 stem and leaves. The flowers, which at first might appear to 

 belong to a third division, are really homologous with leaves 

 and frequently show the relationship by becoming leaflike. 



