54 



BOTANY. 



are no cut places on the shoots or leaves, we infer that the loss 

 of water vapor takes place from the surfaces of the leaves and 

 from the shoots. It is also to be noted that, while this plant is 

 losing water from the surfaces of the leaves, it does not wilt or 

 lose its turgidity. The roots by their activity and osmotic 

 pressure supply water to take the place of that which is given 

 off in the form of water vapor. This loss of water in the form 

 of water vapor by plants is transpiration. 



Synopsis. 



As a result of the law of diffusion by which water from the 

 soil is drawn inside the root hairs forcibly by the cell- 

 sap, and is passed on through the cells of the root by 

 the same law of diffusion, a pressure occurs which causes 

 the liquid plant food to rise to some extent in the roots 

 and stems of plants. 



The height to which water can be lifted by root pressure 

 varies in different plants. 



Root pressure is not constant throughout the day in a 

 given plant, but varies. 



Root pressure is usually lower at night and higher toward 

 midday. 



Plants then show a daily periodicity in the strength of the 

 root pressure, but the periods are not coincident in all 

 plants ; that is, the time of day when one plant shows 

 the greatest root pressure is not necessarily the same for 

 another plant. 



Some plants also show an annua! periodicity in the strength 

 of the root pressure. 

 , Living plants are constantly losing water by evaporation 

 (or transpiration) from the surface, unless the air is sat- 

 urated with moisture. 



If plants are removed from the soil, or shoots are cut away, 

 they "wilt," or become flabby, because of the loss of 

 water. 



This loss of water from plants, or plant parts, can be dem- 

 onstrated by placing the plant under a glass receiver. 



The water escapes in the form of invisible water vapor. 



When the plant is growing normally, the roots by absorp- 

 tion of water from the soil supply water to take the 

 place of that evaporated from the exposed plant surface. 



Root pressure 

 or osmotic 

 pressure. 



Transpiration. 



