PHYSIOLOGICAL PROCESSES DURING FLOWERING 485 



illumination. They are fully expanded therefore in the eveniiifr 

 and in cloudy weather. 



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

 tion. The movements of the compound leaves of wood sorrel 

 (Oxalis), clover, and other plants are thus conditioned. In 

 some of these plants, the leaflets droop during the night; in 

 others, they rise. These movements are produced by means of 

 a special erecting tissue at the bases called the ^^pulvinus." The 

 movements are of an alternating nature and are due to turgor 

 changes in the upper and the lower halves of the pulvinus. 

 Drooping movements are usually accompanied by an increase 

 of turgor in the upper half of the pulvinus 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 flowers under unfavorable conditions.. 

 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 

 presents no danger to the plant, this explanation should he 

 accepted with reservations. 



Of all the nastic movements, the greatest attention has been 

 paid to those of seismonastic nature, which occur in response 

 to a shock or concussion experienced by a plant. 



Such movements may be observed in flowers of many Com- 

 positae, for instance, the cornflower. When touched, the threads 

 of the stamens contract abruptly; this causes a lowering of the 

 tube formed of united stamens surrounding the pistil and the 

 throwing out of pollen onto the head of any insect that introduces 

 its proboscis into the flower. In barberry, the staminal filaments 

 curve rapidly when a special cushion of tissue at their bases is 

 touched, and the stamens strike the pistil. All these movements 

 are connected with the process of pollination of plants. 



For other seismonastic movements not connected with flower- 

 ing, the best-known illustration is supplied by the sensitive 

 plant. Mimosa pudica, which rapidly drops its leaves when 

 touched, all of its leaflets folding together in pairs (Fig. 137). 

 If the plant is severely shaken, then all its leaves will drop at 

 once. A very strong irritation of a separate leaf, as by injury 



