136 



BIOLOGY 



Motion. The most striking distinction ordinarily recog- 

 nized between animals and plants is the absence of the power 

 of motion in plants and its presence in animals. This distinc- 

 tion, however, is by no means a sharp one, for motion is not 

 wholly lacking in plants. Many of the lower types of plants 

 are capable of locomotion. This is confined largely to the micro- 

 scopic forms, and in some plants it is present only in their re- 

 productive spores. For example, Ulothrix (see page 93) is a 

 motionless organism in its ordinary adult form, but produces 

 reproductive spores, called zoospores, which swim rapidly in the 



water. Among other 

 microscopic plants, 

 locomotive power is 

 found even in the adult 

 life of the animal. 

 This is true of Osdl- 

 laria, Diatoms, and 

 some other organisms ; 

 Fig. 68. Among the 

 higher plants no active type of locomotion is found, although 

 many of them are constructed in such a way that they may be 

 carried to and fro by motile animals. Even among the highest 

 plants, however, a certain amount of motion is developed in the 

 different parts of the plant. Among the flowers of the highest 

 groups of plants, motion is developed in certain parts ot the 

 flowers for the distribution of pollen. In most of the highest 

 class also, careful study has shown that the leaves are constantly 

 in a state of slow motion, waving to and fro during the growth 

 of the plant in sunlight. Of course the leaves are almost always 

 moved by the wind, but quite independently of air currents they 

 have a motion of their own which can be detected by a careful 

 recording apparatus. It is thought that this motion is due 

 principally, perhaps entirely, to the unequal evaporation of 

 water on different sides of the stem. At all events it is so slight 

 that it can hardly be considered true motion, and it certainly 



FIG. 68. THREE PLANTS HAVING 



THE POWER OF MOTION 



A and B, Diatoms, which move readily through water; 

 C, Osdllaria, which simply waves back and forth. 



