CREEPING MECHANISMS. 



843 



and how they brush against one another and against the branches of neighbour- 

 ing shrubs as they swing, and thus receive the stimulus necessary to cause them 

 to throw off the fruits. The contact of animals is, however, a more advantageous 

 means of dispersion, inasmuch as the fruits may be left hanging to their coats by 

 the hard styles and the range of distribution be greatly increased thereby. When 

 there is no assistance from animals, and the cast-off fruits simply fall to the ground, 

 the range of projection is not more than 2-3 metres, which is a comparatively small 

 distance from the spot where the fruits were ripened. 



The limitation of the range of dispersion is still more marked in the case of 

 fruits which creep or hop along the ground than in those where the action is that 

 of a sling or of a catapult. The fruits in question have stiff and very hygroscopic 

 bristles projecting on one side from their external coats, and these bristles continually 

 change their position according to the varying state of the environment in respect 



Fii:, 461. — Creeping and hupping fruits. 

 ■ ^gilops ventricosa. 2 ^gilops ovata. 8 Crupina vulgaris. * Trifolium stellatuiib 



of moisture, and by so doing propel the fruit or seeds, as the case may be, in a 

 definite direction. The awns which project from the glumes of Grasses (e.g. Elyrtius 

 crinitus, Secale fragile, and various species of JSgilops; see figs. 461^ and 461"^), 

 the strong bristles in which the bract-scales of the flowers in Restiaceae terminate 

 {e.g. the South African plant, Hypodiscus aristatus), the calyx-bristles and stiff 

 pappus-hairs in Scabiouses and Composites (e.g. Crupina vulgaris, see fig. 461 ^), and 

 the divergent calyx-teeth in Papilionacese (e.g. Trifolium stellatum, see fig. 461 *) 

 constitute structures whereof the different parts alternately approach and recede 

 from one another and so cause a movement resembling that of creeping. In all 

 these cases the hygroscopic structures are furnished with small teeth. Sometimes 

 the teeth are on both sides, sometimes on one side, and sometimes only at the 

 tip (see figs. 462 ^'-•^•^). The teeth render i-etrogression impossible, and to that 

 extent determine the direction in which the fruit moves. In Avena elatior, 

 Avena pratensis, and several other Grasses the awns which project from the base 

 of the enveloping glumes are bent elbow-wise. The part below the bend is 

 spirally twisted, and as the tissue is extraordinarily hygroscopic, the spiral relaxes 

 or contracts according to the amount of moisture in the air. This spiral motion 

 causes the part of the awn which is above the bend to move like the hand of a 

 watch, but now to one side, now to the other. Of course this movement can only 



