122 PHYSIOLOGICAL BOTANY. 



the cell-walls are lignified and made to be easily penetrated 

 by water. 



From the last experiment it will be seen that the root- 

 has itself the function of carrying up the water it has 

 absorbed. This action is called the Boot-pressure. 



As the leaves lose continually water by transpiration, the 

 water absorbed by the root ascends towards them to com- 

 pensate for the loss ; and in the growing portion a fresh 

 supply of water is necessary, and a sufficient quantity is 

 conveyed towards it. 



The movement of water is therefore caused by three dis- 

 tinct actions, namely : the root-pressure ; transpiration ; and 

 growth. 



6. THE MOVEMENT OF GASES. 



In the plant-body, the gases dissolved in water are con- 

 veyed by it to the point which is relatively poor in them ;. 

 and the gases in the intercellular spaces move freely, and 

 communicate with the outer air by means of the stomata. 



7. ASSIMILATION. 



If a water plant as the Sasamo be placed in a glass vessel 

 containing water, which holds carbonic acid in solution, be 

 covered by an inverted glass funnel, whose narrow portion 

 is again covered by an inverted test tube filled with water, 

 and be exposed to sunshine, it will be seen that bubbles of 

 gas are given off from the plant and accumulate witbin the 

 test tube. If the accumulated gas be tested, it will be found 

 to consist of oxygen. 



If water, which does not hold carbonic acid in solution, 

 be used in the above experiment, no bubbles of oxygen gas 

 will be given off. 



If green leaves are cut off from the stem during the day 



