608 PHYSIOLOGY. 



current acting for a long time may result in retarding the movement of 

 the protoplasm, and finally, under certain conditions, in arresting it alto- 

 gether. Weak currents always produce a retardation of the plasma- 

 movement ; and continued for a longer period still, motion may be sus- 

 pended. After the movement of the protoplasm has been rendered 

 slower, it will again in a short time recover its normal degree of motion, 

 provided it has been disturbed by no sudden fluctuations of the electric 

 current. When the movement of the protoplasm is perfectly stopped, 

 without any other essential change, it after a time gradually sets 

 in again if left to itself. The parts of the cell, in most of the plants 

 examined, in which by electrical effect the protoplasm and chlorophyll 

 granules are accumulated are the septa ; but with stronger currents 

 aggregations may appear in various parts of the cell. When once the 

 movement has been checked it only very gradually attains its former rate. 

 By moderate electrical irritation molecular movement is produced. In 

 most cases the different parts of the contents of the cells are unequally 

 affected. Strongly intensified currents deprive the protoplasm of all 

 further power of motion. Very strong currents induce contraction of the 

 primordial utricle. Making a contact has often a greater physiological 

 effect than breaking the contact. The condition of agitation of the pro- 

 toplasm brought about by the electric current is not transmitted to 

 contiguous parts. By weak currents the protoplasm is enabled to take 

 up water through its imbibing channels ; and the water thus taken up 

 may be expressed by the protoplasm itself if left to rest. By moderate, 

 though not too weak, irritation, perfect vacuoles are formed, after which 

 death or resuscitation may follow ; this is the boundary of life and death. 

 By strong electrical currents the protoplasm is enabled to take up water 

 in its own interstices : it swells up. The same property is possessed by 

 the granules of chlorophyll. The effect of strong currents is to cause the 

 separation of solid particles of the protoplasm. Protoplasm and granules 

 of chlorophyll are transformed by electricity into the viscid fluid state of 

 aggregation, and separate parts arrived at this state may flow together. 

 The rotation of the granules of chlorophyll in the cells of Cham is not 

 disturbed by galvanic currents to the same extent as the protoplasmic 

 movements, and may be viewed even when the movement of the proto- 

 plasm has been artificially brought almost to a standstill. Rather strong 

 electric currents in several instances cause a momentary reversion of the 

 circulation ; but this is only apparent because it involves some essential 

 changes. On the application of strong electric currents the protoplasm 

 collects with great f acuity, especially on the cell-wall nearest the one or 

 other pole, in the form of plates or elliptical bodies. 



Gravitation, Geotropism. The effects of gravitation on living plants are 

 ill understood. Knight was the first to show that the downward growth 

 of roots was due to the force exerted by the earth j for when this was 

 removed or supplanted by some force of superior power, downward 

 growth ceased, as when seeds were grown on a rotating wheel, when the 

 roots were directed outward by centrifugal force. Gravitation acts inde- 

 pendently of chemical or other forces, and presents many phenomena 

 strictly analogous to those mentioned under the head of Heliotropism. 

 When a plant is so placed that the amount of light is equal on all sides, 

 or when heliotropic curvatures are prevented, gravitation causes some 



