GROWTH, IRRITABILITY AND MOVEMENT 



165 



Pig. 100. 



Leaf of Dionaea in the receptive state. 

 (After Darwin.) ( x 4.) 



from glandular hairs on the upper surface. The movements of the 

 Venus Fly Trap [Dionaea), an American plant, are very strik- 

 ing. Each of the rosette of 

 leaves of the plant bears at 

 its distal end a two-flapped 

 mechanism, like the covers of 

 a book, mobile along the 

 median line as a hinge (Fig. 

 100). The flaps are furnished 

 with marginal spines, while 

 three sensitive bristles rise 

 erect from the upper surface of 

 each. Under favourable condi- 

 tions, a touch on any of these 

 six bristles results in the immediate closure of the flaps, with interlock- 

 ing of the marginal spines. Any insect touching them would be cap- 

 tured within the trap ; and as the inner 

 leaf-surfaces are furnished with secret- 

 ing glands, digestion follows, with the 

 usual absorption of the soluble products 

 of digestion. The movement is due to 

 combined turgor and growth changes. 



5. Hygroscopic Movements. 



All the instances of movement in plants 

 that we have so far considered, whether 

 tropic or nastic, depend en changes of growth 

 or turgor, and therefore involve living cells. 

 There is, however, another class of movements 

 in plants which stand in no direct relation to 

 living cells, and which are physical rather 

 than physiological in nature. These move- 

 ments can take place in dead plant organs 

 and are due to changes in water content of 

 the tissues, in combination with special struc- 

 tural features. Such hygroscopic movements 

 are seen in the dehiscence of many fruits, and 



Fig. ioi. 



Fruits of Cardamine hirsuta. The upper- are ft e n due to tensions set up through un- 

 most are unripe. Those below them are . . ' ___ . 4-u^ 



ripe, and the carpellary walls, splitting equal contraction of different layers 01 tne 

 away from below, curve so quickly as to ,. f ., f .. ^ dr j es The tensions 



throw the seeds forcibly outwards. wau OI tne Iruit ab 1U U11CS - 



ultimately lead to the rupture of the fruit, 



and in the sudden relaxation the seeds may be thrown out and scattered. An 



instance of this is provided by the native Hairy Bitter-Cress (Fig. 10 1), but a 



more notable one is the Sand Box Tree (Hura crepitans), a native of tropical 



