11 

 Lecture 4, July 9. 



Watgr . 



Water is one of the necessary elements for plant life. Plant 

 st.rxictixre and cell sap both contain much water. Quantitatively not 

 qualitatively there is a third factor in which water is the most 

 important factor, that is in supplying the water lost by transpira- 

 tion. The transpiration current is the principal means of supplying 

 plants with food and water. The old theory was that food material 

 was carried along like leaves on sticks in water. The present 

 theory is that each substance obeys its own law and acts according 

 to the law of osmosis peculiar to itself. This makes each siibsta.nce 

 active instead of passive. In water plants there is no definite 

 current carrying water from part to part. Probably there is no 

 transpiration in water plants. 



Water is intermediate between soil and air. Soil is most 

 stable, air, the least. Air is most transparent, soil least and 

 water intermediate. For plant structure a certain amount of 

 stability is necessary. Transparency is essential to leaf work. 

 7/ater alone is best fitted to support plant lif^. 



Dangers of water relations. 



This danger has only been recently explained. 

 1. Plants can take up water more rapidly than they can givo it 

 out. The power which forces water from root to stem is called 

 root pressure. Plants may take up so much water that transpiration 



