THE MANIFESTATIONS OF LIFE 69 



iation from the part stimulated to all parts of the 

 organism. 



Among such of the higher plants as show response to 

 stimulation we find the effects to result from the sum 

 of the reactions taking place in numbers of cells through 

 irritable impulses transmitted from cell to cell from 

 the point of primary stimulation. For example, when 

 the hairs upon the expanded leaf of Dionoea are irritated, 

 the leaf closes through changes effected in cells remote 

 from the point of stimulation. Indeed, so many cells 

 are affected and the effect of their united activity so 

 pronounced as to lead to a result strikingly dispropor- 

 tionate to the intensity of the stimulation. The same 

 general principle applies to Drosera and other insect- 

 catching plants. A few tentacles being stimulated by 

 the insect, many bend over and assist in catching it. 



Another example in which still more widespread 

 conduction of the impulse from cell to cell is found 

 in Mimosa, for when a single pinnule of one of the leaves 

 is actively stimulated, the whole leaf, or the whole 

 branch, or, indeed, the whole plant, may be so disturbed 

 as to close its leaves. Here we see indubitable evidence 

 that the impulse to react passes from cell to cell along 

 the pinnules, petioles, branches, and stems. 



Among animals conductivity is more easily demon- 

 strated because of the generally greater activity of ani- 

 mal organisms. In plants the signs of conductivity 

 and irritability are most evident among the lowest forms, 

 but in animals they are most obvious and best developed 

 among the highest forms. 



When a moving amoeba is irritated in such manner that 

 a single pseudopod is affected, all of the pseudopods 

 are drawn in, the spherical shape is assumed, and the 

 animal remains quiet for a time. When the delicate 

 pseudopods of any of the radiularia are touched, they 

 may be withdrawn, or all of the pseudopods may be with- 

 drawn as in amoeba. In these illustrations it will be 

 seen that a disturbance at one point is transmitted 



