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TEXTBOOK OF PLANT PHYSIOLOGY 



artificial shading, as for instance, by carrying the plant into a 

 weakly illuminated room. Some plants, like Nicotiana and the 

 four-o'clock, on the contrary, close their flowers when the light is 

 increased and open under weaker illumination. They are fully 

 expanded therefore in the evening and in cloudy weather. 



Often the effect may be due both to temperature and illumina- 

 tion. The movements of the compound leaves of woodsorrel 

 (Oxalis), clover, and other plants are thus produced. In some of 

 these plants the leaflets droop during the night, in others they rise. 

 These movements are produced by means of articulations of 

 the leaves. They are of an alternating nature and are due to 

 turgor changes in the upper and the lower halves of the nodes. 



Fig. 139. — Photonastic opening of a dandelion flower head (from Bonner Lehrbuch). 



Drooping movements are usually accompanied by an increase of 

 turgor in the upper half of the node and a decrease in the lower 

 half, while a lifting movement is produced by a reverse mechanism. 



Nyctinastic movements of plants have an ecological role. 

 In flowers, they facilitate pollination in favorable weather and 

 protect the inner organs of blossoms during unfavorable condi- 

 tions. Nyctinastic changes in position of leaves are thought to be 

 of importance as a protection against chilling at night. But since 

 they are most common in the tropics where cooling at night pre- 

 sents no danger to the plant, this explanation should be accepted 

 with caution. 



Of all the nastic movements, the greatest attention has been 

 paid to those of seismonastic nature, which occur in response to a 

 shock or a concussion experienced by a plant. The best-known 

 illustration is supplied by the sensitive Mimosa pudica, which 

 rapidly droops its leaves when touched, all of its leaflets folding in 



