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 



DUCHABTRK. 



