MOVEMENT 367 



Among these autonomic growth movements are the nutations 

 discovered and described by Darwin. The tips of many stems do 

 not grow the same on all sides or at the same rate, with the re- 

 sult that the tip nods successively to all points of the compass 

 describing a circle or an ascending spiral as the stem grows in 

 length. In the case of twining stems, these nutations are very 

 striking. 



Nastic movements of an autonomic nature are seen in the 

 opening of buds. When the primordia have reached the proper 

 stage of development, the upper half of the bud scales grows more 

 rapidly than the lower side and the scales turn back. Similarly at 

 the opening of the flower, the upper half of the growing base of 

 the petals and sepals grows more rapidly than the lower side 

 and the flower opens. In some cases, as mentioned above, these 

 changes are reversible and are connected chiefly with variations 

 in turgor. 



Paratonic Growth Movements. — These movements include 

 the tropisms which differ from the nastic movements in two par- 

 ticulars. In the tropisms the direction of movement and growth 

 depends upon the direction of the stimulus, while nastic move- 

 ments depend upon changes in intensity rather than in direction. 

 Secondly, tropisms occur chiefly in organs which are radially 

 symmetrical and free to move in all directions, while nastic move- 

 ments are found only in parts which, owing to their structure, 

 are free to move in only one plane. Nastic movements are found 

 consequently chiefly in leaves, petals, etc., where they occur in 

 a vertical plane. Tropisms, however, occur in any plane, and 

 even organs like leaves, which are not radially symmetrical, may 

 exhibit tropistic movements. 



Phototropism.— Of all the external factors which influence the 

 growth and development of plants,, light is one of the most im- 

 portant, as has been emphasized already many times; and in view 

 of this previous work, the value of the responses here described 

 will not be difficult to understand. 



Stems are generally positively phototropic, i. e., they grow 

 toward the light. The growth on the side nearest the light is 

 checked, with the result that bending occurs. In a few cases (see be- 

 low) the stems are negatively phototropic, but such cases are rare. 



As in geotropism, there is a perceptive region at the tip of the 

 stem, but it is larger and less localized than the perceptive region 



