SENSITIVITY IN PLANTS AND ANIMALS 73 



of the cells are in a continual state of movement. It 

 is, of course, the protoplasm which moves, and in 

 doing so carries with it the chloroplasts, and by 

 watching these the rate of motion of the protoplasm 

 may be, at all events approximately, measured. If 

 such a leaf be exposed to ether vapour, the streaming 

 gradually comes to a standstill, to be resumed when 

 the ether vapour is removed, unless the exposure to 

 the vapour has been too prolonged, under which 

 circumstances the protoplasm is paralysed. 



Again, if we examine some of the extremely minute 

 organisms known 

 as Bacteria, we 

 find that, in the 

 motile state, they 

 are sensitive to 

 the presence of 

 oxygen gas, being 

 attracted to it 

 wherever it is pro- 

 duced. We have 

 already seen that 

 a green cell in 

 sunlight and in the presence of carbon dioxide manu- 

 factures organic substances and evolves oxygen gas 

 during the process. Let us place such a green cell, 

 say, of a unicellular plant, in the centre of a cover- 

 glass preparation in water (Fig. 30). Obviously, 

 if exposed to light under the microscope, oxygen will 

 be given off from it and will accumulate in the water 

 in the immediate vicinity of the cell. If we introduce 

 some motile Bacteria below the cover-glass they will 

 aggregate round the green cell. If the preparation 

 be darkened for a time and then examined, we shall 

 find that most of the Bacteria have now betaken 



FIG. 30. Bacteria and green cell. A, ex- 

 posed to light ; B, in darkness. (After 

 Engelmann. ) 



