OSMOSIS AND SAP-PRESSURE 



73 



placed in weak sugar solution (each cell having a concen- 

 tration greater than the outside solution), it takes up water. 

 The slices of tuber in the strong solution lose water because 

 the concentration of the external solution is stronger than 

 that of the cell-sap. 



152. The root-hairs are able to take up water from the 

 soil because the soil solution is extremely dilute. If the soil 

 solution were strong, the plant might give up 

 water to the soil. It would be possible to add 

 so much fertilizer to the land as to cause the 

 plant to lose water by exosmosis. There is 

 seldom, however, any danger that the farmer or 

 gardener will add so much fertilizer to the soil, 

 in practice, as to cause a wilting of the plant 

 due to loss of water by exosmosis. 



153. The water and salts in solution taken 

 up by the root-hairs pass into the root proper 

 and finally into definite routes that are con- 

 tinuous from the root through the stems to the 

 leaves. To illustrate the path of water-ascent, 

 insert a growing shoot in water that is colored 

 with eosin. (Eosin may be had of dealers in 

 microscopic supplies. Common aniline may 

 answer very well.) The tissues stained with 

 the dye are the conducting tissues. In woody 

 plants, the water is conducted in the young 

 wood, not between the bark and wood 

 as commonly supposed. 



154. The absorption of water by a 

 root may be so rapid as to give rise to 

 distinct pressure. This force is root- or 

 sap-pressure. It varies in different plants 

 and in the same plant at different times. 

 The "bleeding" of plants is a manifesta- 

 tion of this pressure. In the spring, the 



124. To show 

 sap-pressure. 



