308 



PLANT STRUCTURES 



foliage leaf, stamen, etc. It is interesting to note that these 

 movements have been cultivated by but few families, nota- 

 ble among them being the Legumes ( 141). 



These movements of mature organs, some of which are 

 very rapid, are due to changes in the turgidity of cells. As 

 already mentioned ( 157), turgid cells are inflated and 

 rigid, and when turgidity ceases the cells collapse and the 

 tissue becomes flaccid. A special organ for varying tur- 

 gidity, known as the pulvinus, is usually associated with 

 the motile leaves and leaflets. The pulvinus is practically 

 a mass of parenchyma cells, whose turgidity is made to vary 

 by various causes, and leaf -movement is the result. 



The causes which induce some movements are unknown, 

 as in the case of Desmodium gyrans (see Plant Relations, 

 p. 49), whose small lateral leaflets uninterruptedly de- 

 scribe circles, completing a cycle in one to three minutes. 



In other cases the inciting cause is the change from light 

 to dark, the leaves assuming at night a very dif- 

 ferent position from that during the day. Dur- 

 ing the day the leaflets are spread out freely, 



FIG. 280. A leaf of a sensitive plant in two conditions: in the figure to the left the leaf 

 is fully expanded, with its four main divisions and numerous leaflets well spread; 

 in the figure to the right is shown the same leaf after it has been " shocked " by 

 a sudden touch, or by sudden heat, or in some other way; the leaflets have been 

 thrown together forward and upward, the four main divisions have been moved 

 together, and the main leaf-stalk has been directed sharply downward. After 



DUCHARTRE. 



