THE PLANT-CELL 43 



able to the so-called eye-spot of some of the lower animals. The 

 well-known effect of the intensity of light upon the movement and 

 rate of growth in the organs of the higher plants is necessarily con- 

 nected with the behavior of the protoplasm in the cells of the 

 growing part. 



Heat. Below a certain temperature, which varies much in differ- 

 ent cases, the activity of the protoplasm stops. Very few plants 

 show activity when the temperature falls below the freezing point 

 of water, but they are not necessarily killed at this temperature. 

 As the temperature rises, there is an increase in the activity of the 

 protoplasm, especially evident where movements are present, but 

 this continues only up to an optimum temperature varying iii differ- 

 ent cases. Above this optimum the protoplasmic activity decreases 

 rapidly, and finally ceases entirely. The albuminous substances co- 

 agulate, and the protoplasm dies. Since some organisms, like Bac- 

 teria and allied forms, can endure a temperature nearly or quite up 

 to the boiling point of water, it is evident that in these forms the 

 albuminous protoplasmic constituents must be modified, as the ordi- 

 nary proteids coagulate at a much lower temperature. 



Electricity. In general, the effect of electric currents passing 

 through protoplasm is to cause contraction and a cessation of move- 

 ment. Long-continued currents finally result in a complete disor- 

 ganization of the protoplast. In free-swimming cells, where the 

 current is not too violent, there is a tendency for the cell to move 

 toward the negative pole. 



Mechanical Stimuli. An Infusorian or other naked protoplasmic 

 mass, on being touched, will contract strongly, and the same effect 

 is seen when the water is agitated. Where the protoplasm is within 

 a cell-wall, the movements of the currents are checked, or completely 

 stopped, by a violent shock. If a hair is torn off from a stem and 

 examined with the microscope, it does not at first show the stream- 

 ing movements, which are only resumed after it has recovered from 

 the mechanical shock. 



Chemotaxis. Various chemical substances exercise a powerful 

 influence upon protoplasm, seen especially in the directive power 

 in its movements. Bacteria collect in great numbers about Algae 

 which are giving off oxygen, and the Bacteria serve as a very deli- 

 cate test of the amount given off at different points. The motile 

 male cells, or spermatozoids, of Ferns have been shown to be strongly 

 attracted by a dilute solution of malic acid, and other organic sub- 

 stances have been shown to exercise an attraction on many organisms. 

 This sensitiveness to chemical influences has been called Chemotaxis. 



Hydrotropism. As might be expected, the presence of water 

 affects the movements of protoplasm. A well-known example is the 

 behavior of the plasmodium of the Slime-moulds. If placed in the 



