366 VEGETABLE PHYSIOLOGY 



for it involves a particular condition of the protoplasm 

 with regard to its permeability. This condition may be 

 regarded as a kind of effort, the living substance exerting 

 some active living influence comparable to the condition 

 of almost passive contraction, which is the normal condition 

 of various muscular structures in the animal body. The 

 effort seems to be directed to reducing the resistance its 

 structure offers to the passage of water through it. If it 

 is increased, the existing hydrostatic pressure causes an 

 excessive escape of water, and the cells become flaccid ; if 

 it is relaxed, the normal interchange of water between cells 

 is diminished to their detriment, the permeability of the 

 protoplasm becoming lessened. 



A further aspect of tone may be seen to depend upon a 

 constant and regular supply of oxygen. The function of 

 this gas in vegetable life has already been discussed at 

 some length in a preceding chapter. We have seen that 

 if its access is interfered with the whole organism is for a 

 time, if not permanently, upset, all the vital functions 

 being thrown into disorder. The power of appreciating 

 and responding to stimulation is also lost. 



Another property which vegetable protoplasm possesses, 

 and which is of the highest importance in adapting the 

 organism to its environment, is what has been termed 

 acclimatisation. This is manifested by the fact that after 

 long continued applications of a particular stimulus the 

 organism ceases to respond to it. This is shown by the 

 fact that a plant, accustomed to live in light of but feeble 

 intensity, if made to grow in a brighter region, though 

 injuriously affected at first, will ultimately thrive in it as 

 well as it did before. Similar phenomena in connection 

 with temperature have been observed. 



