DIV. ii PHYSIOLOGY 327 



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. 



Many perennial plants do not reappear in exactly the same spot. 

 Since new parts arise by growth while old portions die off, such plants 

 change their place gradually. A good example is afforded by plants 

 with rhizomes growing forward at the tip while the hinder region is 

 decaying. In trees the main axis continues alive, but the growing 

 points are changing their position ; thus the growing point of a giant 

 Australian Eucalyptus moves from the level of the soil to a height 

 of 110m. 



Geophytes (p. 177) moving forward in a straight line in the ground come to 

 new places, the food materials of which have not been used up by them. This 

 movement is especially evident in those in which the rhizome remains short, owing 

 to the former year's growth soon decaying. The annual movement in Listera ovata 

 is only 3-5 mm., in Arum maculatum 1-3 cm., and in Paris quadrifolia 6-8 cm. 

 The change of place is more marked in the case of plants provided with special 

 off-shoots or runners. The movement is not really wanting in geophilous plants 

 which continue in the same spot (Ophrydeae), because the direction of elongation 

 regularly alternates, or as in Colchicum because the new shoots in their expansion 

 have to force their way through the remains of the previous year's growth. In the 

 latter case the corms may be laterally distant, 5-7 cm., from the foliage leaves and 

 connected with these by means of an S-shaped curved stem. 



v In addition to these movements, occasioned by a growth in length, 

 plants firmly established in the soil also possess 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 curved that they break through the soil without injury 

 to the young leaves. 



Movements of locomotion and movements of curvature have thus 

 to be distinguished. 



I. MOVEMENTS OF LOCOMOTION ( 88 ) 

 A. Mechanism of Movements of 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 



