292 BOTANY pakt i 



a gradual change of position ; but cells enclosed by cell walls possess 

 also the power of independent locomotion, often indeed to a con- 

 siderable extent. Multicellular plants, however, as a rule ultimately 

 attach themselves, by means of roots or other organs, to the place of 

 germination, and so lose for ever their power of locomotion, except in 

 so far as it results from growth. A gradual change in jaosition due to 

 growth is apparent in plants, the rhizomes and stems of which are 

 continually growing forward. Thus the growing point of the giant 

 Eucalyptus tree of Australia is moved from close to the soil to a height 

 of 100 m. The tip of a rhizome may move more slowly, but perhaps 

 in the course of centuries, even further. In addition to these move- 

 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. Not only unequal growth 

 but other processes also take part in these changes of form. In this 

 way the organs are brought into positions necessary or advantageous 

 for the performance of their functions. 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 and tendrils 

 twined about a support, and the stems of seedlings so bent that they 

 break through the soil without injury to the young leaves. 



A. Locomotion 



In a fuller consideration of changes of position we can leave on 

 one side the carriage forward in a straight line by means of growth of 

 the growing point since this has been dealt with in the chapter on 

 development. We thus confine ourselves to the protoplasmic move- 

 ments among which the amoeboid movement, the ciliary move- 

 ment, and the movement of protoplasm in cells with cell-walls 

 may be distinguished. 



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, are distinguished 

 as amoeboid 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 to surface tension, which the proto- 

 plasm can at different jjoints increase or diminish by means of its 

 quality of irritability. 



By means of irregular clianges of surface-tension similar amoeboid movements 

 are also exhibited by dro])s of lifeless fluids, such as drops of oil in soap solution, 

 drops of an oily emulsion in water, or drops of mercxu'y in 20 per cent solution of 

 potassium nitrate in contact with crystals of potassium bichromate. 



In the swiMMiNd movements by means of cilia, on the con- 



