CHAPTEE XL 

 THE MOTOR SYSTEM. 



I GENERAL CONSIDERATIONS. 



Movements analogous to those of animals, consisting in a change in 

 the space-relations of the whole organism, or of some of its organs, are 

 of widespread occurrence among plants ; such movements are generally 

 adaptive in character. An important distinction between the two 

 classes of living beings consists in the fact that movement of the 

 entire organism that is, locomotion is the rule among animals, but 

 the exception among plants. Movements of individual organs, 

 carried out in the interests of their own physiological activity, or for 

 the benefit of other organs, on the other hand, are of general occurrence 

 in the vegetable kingdom. 



It is well known that a number of the lowest plants are freely 

 motile throughout life, or, at anyrate, at certain stages of their life- 

 history. Such cases show that, in spite of what has just been stated, 

 we cannot invariably rely upon the prevailing type of movement in 

 order to distinguish a plant from an animal. Several distinct modes 

 of locomotion are known, and more than one form of special locomotor 

 organ has been evolved for the purpose of carrying out these 

 movements. Three principal types of active locomotion may be 

 distinguished. These are exemplified respectively by the amoeboid 

 creeping movements of Myxomycete plasmodia, the gliding movements 

 of Diatoms, Desmids and Oscillatoeieae, and the swimming movements 

 of Bacteria, Volvocineae, and swarm-spores, which depend upon the 

 action of vibrating or undulating flagella. Among Higher Plants, on 

 the other hand, active locomotion is unknown ; the only movements of 

 translation that occur, are the gradual progression of growing parts of 

 the plant-body, and the passive transportation which is almost entirely 

 restricted to seeds and fruits. The last mentioned movements often 

 depend upon the presence of special organs which provide points of 



