380 The Living Plant 



though slowly, by the simple device of directing their protoplasmic 

 streaming in one constant direction, precisely as is the habit of the 

 much smaller Amceba among animals (figure 147). Other plants, 

 again, or their reproductive spores, can swim freely in water, in a 

 manner so like that of animals that 

 they are known as " animal spores," or 

 zoospores; and these are very abundant 

 and characteristic in the Algae or Sea- 

 weeds (figure 94). The motion is ef- 

 fected either by the action of innumer- 

 able cilia, tiny hairs which all in unison 



FIG. 147. An Amoeba, greatly , . 



magnified; a creeping organism beat the water more strongly in one 

 direction than the other, or else by 



flagellae, which are structures suggestive of tails, except that 

 instead of pushing the spore, they pull it behind them by an 

 action the reverse of the one used in the tail of a fish. These 

 movements of flagellae and cilia, by the way, depend upon the 

 power of contractility in protoplasm, a form of the motility 

 which has already been described as one of the physiological 

 properties of that substance; and this same contractility, also, 

 is the basis of the muscular mechanism of the higher animals. 

 Other kinds of water plants, notably the Blue-green Algae, make 

 use of vibration of their rod-like bodies, securing their movement, 

 I suppose, in essentially the same way that a piece of flexible steel 

 is shot through the air after having been bent between thumb 

 and forefinger and then quickly released. Other kinds push out 

 protoplasmic threads, which work against the bottom, as is the 

 way with the Diatoms, those tiny plants whose wonderfully- 

 sculptured shells are the favorites of every happy possessor of a 

 first microscope. In all of these modes of locomotion, the re- 

 semblance to animals is not accidental, but a persistence from an 

 ancient condition in which the two kingdoms were one. 



2. Extension through Growth. The stems of the higher plants, 

 as the reader will recall, are usually so made that elongated inter- 



