sect, ii PHYSIOLOGY 241 



vitality and organic imperishability, is the substance of the lowest 

 unicellular organisms, continually reproducing themselves by division 

 and ever changing into new individuals. 



V. The Phenomena of Movement 



In every living organism there is constantly occurring in the 

 course of the metabolic processes an active movement and transposi- 

 tion of substance. As these movements are for the most part 

 molecular they are generally imperceptible ; but that they actually 

 take place is demonstrated with absolute certainty by the local 

 accumulation and diminution of substance, shown both by weighing 

 and by the results of chemical analysis. 



There are also other forms of movement which play an important 

 part in the physiology of every organism, and on which its vital 

 processes are to a large extent dependent : these are the movements 

 due to heat and the related conditions of vibration resulting from 

 light, electricity, etc. 



Apart from the movements of this class, which may take place 

 within organisms which, externally, are apparently at rest, there occur 

 also in plants actual CHANGES in position, externally noticeable but 

 usually of gradual operation ; yet in special cases they may involve 

 rapid motion. These movements may be carried on either by the 

 whole plant or by single organs. Reference is here made only to the 

 spontaneous movement resulting from the activity of a plant organism 

 itself, and this should not be confused with the PASSIVE movements 

 due to externally operating mechanical agencies, such as water and 

 wind, which, although they have a certain importance for plant life, 

 will not be here considered. 



Protoplasm itself is capable of different movements. Naked 

 protoplasmic bodies almost always show slow movements resulting in a 

 gradual change of position ; but cells enclosed by cell walls possess also 

 the power of independent locomotion, often indeed to a considerable 

 extent. Multicellular plants, however, as a rule ultimately attach 

 themselves, by means of roots or other organs, to the place of germina- 

 tion and so lose for ever their power of locomotion, except in so far 

 as it results from growth. A gradual change in position due to 

 growth is apparent in plants provided with rhizomes, the apical ex- 

 tremities of which are continually growing forward, while the older 

 portions gradually die off. A yearly elongation of 5 cm. in the 

 apical prowth of the rhizomes would result, in twenty years, in 

 movin"- the plant a distance of one metre from its original position. 

 A seedlino- of Cuscuta in its search for a host plant illustrates the power 

 of maintaining, for a time, a creeping movement over the surface of 

 the soil • a growing Caulerpa (Fig. 250) likewise exhibits in the course 

 of years a similar advancing movement. In addition to these move- 

 it 



