io<S 



BOTANY OF THE LIVING PLANT 



Leaf. Stem 

 and Root 



r 



Root 





J 



phylL 



V n mentioned that 600 lb ter may he evaporated 



a the in t! ne day. Clearly, very 



ill be needed to rai 1 quickly from the 



jnsl ,; ravity and to overcome internal 



leration applies in leaser degree to smaller 



pk, ting to this end in the plant 



much discussion, and although in the Cohesion 

 , The a fairly satisfactory explanation is 

 » now forthcoming, there is admittedly much 



ill to be elucidated. 

 N In the region where transpiration occurs 

 tin- cell-walls bordering on the air-spaces 

 lost ter by transpiration. It may be 



nmed that the walls imbibe a corres- 

 ponding amount of water from the proto- 

 plasts which they enclose, creating in each 

 protoplast an increased osmotic suction 

 which leads it to absorb water from a 

 neighbouring cell. This cell in turn absorbs 

 water from a third cell, the process being 

 repeated until we arrive at a cell which 

 is in contact with a tracheide or vessel 

 of one of the fine leaf-veins. This cell 

 takes water from the xylem element (Fig. 



72). 



At this point reference must be made 



to the Cohesion of water. Because of its 



h« path of the liquid nature, the water under experiment 



Fie. 72 



i> 



throuch 



. trum Mr.i>- must be supported in a rigid envelope be- 

 fore this property can be demonstrated. 

 a suitable arrangement. Before being sealed, the 

 « urved glass tube (actually about one metre in length) is partly filled 

 1 water, which is then boiled so as to wet the sides of the tube 

 thoroughly and to expel the air. The tube is then sealed-off. If 

 it 1. a properly set up, the tube can be brought by careful tilting 



into the position shown, with the water hanging in the long arm. 

 The film of water in contact with the glass clings to it, while the 

 • of the water retains its continuity as the result of its cohesive 

 power The cohesion is due to the mutual attraction between the 

 mole- ules of water, and the weight of the column of water is insuffi- 

 cient to pull the moleculi : .rt. Other experiments have proved 



