PLANTS IN RELATION TO SOIL, LIGHT, AIR 61 



All plants give off moisture from their leaves. The rate 

 of evaporation depends upon the temperature, amount of -ivind, 

 and the amount of moisture which the plant can secure through 

 its roots. Growth is measured by the amount of water used 

 or given off through the leaves. Plant-food comes along with 

 moisture taken up by the roots. If evaporation is checked 

 by cold weather there will be less plant-food used and the 

 growth is checked. 



5. Osmosis. — Place a few slices of dried apples in water. In 

 an hour or so, they become more rigid because of the water taken 

 into the cells. At the same time, place a few slices in strong salt 

 water. Note the difference in time required to swell the slices in 

 brine. Perhaps they will not swell at all in the brine. 



The process by which the moisture enters the cells of the 

 fruit or enters the roots of plants is called osmosis. Much 

 of the liquid outside the cells enters the roots, and a little of 

 the sap in the roots passes into the soil at the same time. 

 Osmosis is the trading of two liquids through the membrane 

 surface. The liquid of the plant is denser than water ; but the 

 sap may not be denser than strong brine. In the trade of the 

 two liquids in any case, a very little of the denser liquid is 

 exchanged' for a large quantity of the thinner liquid. 



The exercise may be varied by using a few slices of wilted 

 apple or wilted potato, if dried fruit is not readily available. 

 (See also the experiment shown in Pig. 39.) 



When soil water is made too dense by strong applications 

 of fertilizer too near the plant, it will cause the wilting of the 

 plants. It defeats the purpose desired by the gardener. Fer- 

 tilizer should be mixed thoroughly with the soil a little ways 

 from the seeds and plants, not too near them. 



6. Overfeeding of Plants. — Take two plants of the same kind 

 from the soil of a window box or garden. Two large lettuce plants 

 would do. Place one in a tumbler of water and the other in a strong 

 solution of sugar syrup. After a few hours, the first is observed 

 to remain fresh and the second has wilted. 



