336 TEXTBOOK OF PLANT PHYSIOLOGY 



transverse geotropism, which changes in the more mature parts 

 into negative geotropism. If a twining stem remains without a 

 support, it stretches and straightens out the previously formed 

 curvatures. But if it has clasped around a support, the coils of 

 the spiral will cling closely to it, due to the same kind of stretching. 

 The importance of geotropism in twining plants is indicated by the 

 fact that they do not form spirals when placed on a klinostat. 

 Moreover, they are able to twine only around a vertical or slightly 

 inclined support, but never around a horizontal one, as the rotation 

 of the tip is always in the horizontal plane. 



The twining and climbing plants of the temperate zone are 

 usually herbaceous annuals. They are of a comparatively small 

 size, the only exception being the hop and the clematis. In trop- 

 ical and subtropical countries may be found many woody plants of 

 this type, the so-called lianas, which closely entangle trees, thus 

 making the tropical jungles quite impassable. 



98. Nyctinastic and Seismonastic Movements. — By tropisms 

 are meant such growth movements as are produced by a unilateral 

 effect of some external factor. They usually result in a definite 

 orientation of an organ with respect to the stimulus, causing most 

 frequently either a positive or a negative curvature. The term 

 "nasties" is applied to such movements that are produced by 

 stimuli, which have no definite direction, resulting in no particular 

 orientation of the plant. l Such stimuli are, for instance, changes in 

 temperature or illumination. A dorsiventral or bilaterally sym- 

 metrical structure of the curving organ is indispensable for move- 

 ments of a nastic nature. Movements involving changes in turgor, 

 otherwise called alternating movements, are here of far greater 

 importance than in tropisms. 



Nyctinastic movements caused by succession of night and day 

 are most common. Many flowers open in the morning and close 

 for the night; some leaves, particularly the compound leaves of 

 papilionaceous plants, change their position during the night and 

 day. These movements occur with considerable regularity, which 

 led Linnaeus to attempt to construct a "floral clock," by 

 making a bed of plants in which the different flowers would open 

 and close at various hours of the morning and evening. Move- 

 ments of this nature are often said to be connected with the 



1 In nastic movements, the direction of the stimulus does not determine 

 the direction of response. 



