ADAPTATIONS TO EXTERNAL MECHANICAL FORCES. ()77 



almost " snap " together like a rat-trap when one of the three bristles on 

 the blade is touched. (Fig. 289.) 



Again, the mechanical movements of the leaflets and petioles of Sensi- 

 tive Plants are well known. They are due to a disturbance in the 

 equilibrium in consequence of water being discharged from the cells of 

 the thick " puhinus " at the base of the petiole into the intercellular spaces 

 on the underside, w^hich causes the loicer half of the tissue of the petiole 

 to collapse, with the resulting fall of the petiole. The cells of the lower 

 half of the cortex have their walls not so thick as those of the upper half 

 above the central cylinder" of wood-fibres, &c., which play no 

 immediate part in the movement. The fall of the petiole is therefore 

 simply due to a diminution of tension of water which kept the petiole 

 €rect, and this is caused, as stated, by loss of turgidity. Water escapes 

 from the cells into the intercellular spaces and then out of the motor 

 organ altogether, equilibrium is overthrown and the petiole falls. 



It is a precisely similar means by which the two halves of the leaf of 



Fig. 289. — Foliage of Venus' Fly-trap {Dioncea Musciimla), 

 (After Le Maout and Decaisne.) 



Venus' Fly-trap {Dioncea Muscipida) (fig. 289) close. When the leaf is 

 horizontal the fibro- vascular bundles spread horizontally through an upper 

 and a lower layer of cellular tissue. It is the upper one only which is 

 concerned ^^dth the motion. The free bristles have their bases of 

 cellular tissue in continuation with the layer. When they are touched 

 turgidity gives way, and the now more turgid lower layer forces up the 

 upper and weaker one till it meets the opposite lobe. Hence, the move- 

 ment itself is purely mechanical. 



The question arises, How is the water discharged? Each cell consists 

 of protoplasm covered by cellulose, the latter being an elastic coat which 

 presses on the protoplasm within it ; but this has the property of absorb- 

 ing water to its fullest capacity, so that in the unexcited state the cells 

 are in a condition of unstable equilibrium, for the cell-wall and its enclosed 

 protoplasm are in a constant state of antagonism. 



When the protoplasm suddenly loses its power to retain the water, the 



