GROWTH AND MOVEMENT 



133 



may be stimulated from without, as in the various paratonic move- 

 ments. A reasonable probability may even be established as to the 

 way in which the stimulus reaches the protoplast, as in the statolith- 

 theory of starch-grains. But still the living protoplast is the active 

 agent. 



There is, however, another class of movements in Plants which stand 

 in no direct relation to the living protoplast. Some of these depend upon 

 the hygroscopic qualities of mature 

 cell-walls, which swell unequally with 

 moisture. This is seen in the awns of 

 certain Grasses, such as Stipa ; or in 

 the peristomes of Mosses, which twist 

 with screw-like movements under 

 changes of the degree of moisture in 

 the air (see Chapter XXII.) . If tissues 

 composed of such twisting cells are 

 closely related, as in a woody fruit, 

 strains may be generated as the fruit 

 ripens, which may be relieved by 

 sudden rupture. This is seen in 

 many explosive fruits, such as the 

 Hairy Bitter-Cress (Cardamine hirsuta} 

 (Fig. 93.) A very notable example of 

 this is seen in the Sand-box Tree (Hura 

 crepitans], the fruit of which explodes 

 with a report like a pistol-shot, and 

 throws the large seeds out to a distance 

 of many yards (Fig. 94). Other cases 

 of movement are due to changes in 

 the volume of water in the cavities of 

 special motor-cells, usually of diminution due to evaporation, result- 

 ing in their change of form. This is well seen in the annulus of Fern 

 sporangia (Fig. 95), which curves backwards as its cells dry, but 

 recovers with a sudden jerk, throwing out the spores. A somewhat 

 similar explanation is given for the case of many Grass-leaves, which 

 curl up under conditions of drought, but uncurl when water is 

 plentiful (see p. 156). All such movements appear to spring from 

 physical rather than from vital-physiological causes. 



The movements seen in Plants are thus varied in their character 

 as they are also in their biological effectiveness. Undoubtedly the 

 most important, as they are also the most general, are the movements 



FIG. 93. 



Fruits of Cardamine hirsuta. The upper- 

 most are unripe. Those below them are 

 ripe, and the carpellary walls, splitting 

 away from below, curve so quickly as to 

 throw the seeds forcibly outwards. 



