242 BOTANY 



ments, 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 twined about a 

 support, and the stems of seedlings so bent that they break through 

 the soil without injury to the young leaves. 



Movements of Naked Protoplasts 



The creeping movements of naked protoplasts, such as are shown 

 by an amoeba or plasmodium, in the protrusion, from one or more 

 sides, of protuberances which ultimately draw after them the whole 



protoplasmic body, or are themselves 

 again drawn in (Fig. 198), are distin- 

 guished as amceboid movements. These 

 movements resemble, externally, the 

 motion of a drop of some viscid fluid on 

 a surface to which it does not adhere, and 

 are chiefly due, according to Berthold, to 

 superficial tension, which the protoplasm 

 can at different points increase or diminish, 

 by means of its quality of irritability. 

 (By means of irregular changes of surface- 

 tension similar amoeboid movements are 

 also exhibited by drops of lifeless fluids.) 



Fig. 198. — Amceboid movement. The T , n 



arrows indicate the direction and In the SWIMMING MOVEMENTS BY 



energy of the movement; the crosses, MEANS OF CILIA, On the Contrary, the 



the points at rest At the time whole p rotop l asmic tod y is not involved, 



bemg the principal movement is . - 1 r ■ i n • 



from H to V, but at any moment it DUt it possesses Special Organs 01 motion 



may be towards R or L, and so i n the form of whip-like FLAGELLA Or 



change the direction of the course „ rrr , mi i r. 



taken by the amoeba. CILIA - TheSe ™Y be 0n e, tWO, four 



or more in number, and arranged in 

 various ways (Figs. 69, 70). They move very rapidly in the water 

 and impart considerable velocity to the protoplast, often giving it at 

 the same time a rotatory movement. While the swiftest ship requires 

 10-15 seconds to travel a distance equal to its own length, the velocity 

 with which these protoplasmic bodies are impelled by their cilia, in a 

 second, is two or three times their length, although, owing to their 

 diminutive size, the distance travelled by them in an hour would 

 amount to only about a metre. The protoplasmic body is conveyed 

 by the motion of the cilia in a definite direction, which is so regulated 

 by the action of stimuli that it may be instantly changed. In this 

 way the direction and velocity of the ciliary movements are made 



