264 BOTANY PART I 



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 ; 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 con- 

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

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

 germination, 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 

 extremities of which are continually growing forward, while the older 

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

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

 moving the plant a distance of one metre from its original position. 

 A seedling of Ouscuta 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. 295) likewise exhibits in 

 the course of years a similar advancing movement. In addition to 

 these movements, occasioned by a growth in length, plants firmly 

 established in the soil possess also the power of changing the position 

 and direction of their organs by means of CURVATURE and ROTATION. 

 In this way the organs are brought into positions necessary or 

 advantageous for the performance of their function. By this means, 

 for example, the stems are directed upwards, the roots downwards ; 

 the upper sides of the leaves turned towards the light, climbing 

 plants and tendrils twined about a support, and the stems of 

 seedlings so bent that they break through the soil without injury to 

 the young leaves. 



