MOTION AND LOCOMOTION 79 



motion among the higher plants when it does occur 

 is purely physical, and dependent on the absorption 

 and evaporation of water and the consequent bending 

 and unbending, extension or shrinkage of parts of 

 the organism, and not, as in the animal, on the 

 movements of special contractile tissues. Further, 

 locomotion in the higher plant is not associated 

 with the problem of self-nutrition, but with the 

 distribution of offspring. On the other hand, 

 many of the lower plants have the power of loco- 

 motion, but these plants are aquatic and their 

 powers of movement are as much associated with 

 the problem of dispersal of progeny as with that of 

 nutrition. 



It has already been said that all plants and animals 

 exhibit powers of motion in some degree. Even in 

 the higher plants the protoplasm of the cells, at 

 least in the young state, shows power of movement ; 

 the leaves of many plants are able to open and close, 

 according to certain conditions ; fruits and seeds, 

 growing points, &c., show powers of movement 

 associated with growth conditions. Illustrations 

 of the power of movement in individual parts of 

 the animal body, apart from locomotion of the 

 whole organism, are too familiar to require citation. 



The types of movement that are exhibited by 

 protoplasm itself are very varied in character. Move- 

 Apart from the circulation of protoplasm in cells, roto S - f 

 already referred to (p. 73) we have the ciliary plasm, 

 movements of the cells lining various tubes, such as 

 those of the respiratory organs, the cells covering 

 the surfaces of gills, &c., and the amoeboid movement 

 of the leucocytes of the blood, of many gland 

 cells, of the cells lining the alimentary canal of 

 many of the lower animals, and, in the plant world, 



