860 



THE DISPERSION OF SPECIES BY MEANS OF FRUITS AND SEEDS. 



levers. The fruits are speedily raised by this means above the edge of the open 

 involucre to a sufBcient height to expose them to the wind. In some other Com- 

 posites, such as the Dandelion {Taraxacum), the fruits do not detach themselves 

 spontaneously from the floral receptable when they are ripe. The segments of the 

 involucre close together in wet weather, as do likewise the hairs or plumes of the 

 pappus. In dry weather the involucre opens, whilst the feathers of the pappus 

 diverge so as to assume the form of a parachute, and in that condition offer a com- 

 paratively large surface for the wind to act upon. A moderate gust of wind is now 

 able to lift the fruits, with their expanded parachutes, off the receptacle and carry 

 them away (see fig. 471 ^). If no breath of wind stirs, they remain upon the 

 receptacle; the damp atmosphere of evening causes both parachutes and involucre 



Fig. 474.— Dispersion of fruits and seeds by the wind. Fruits of a Thistle {Cirsium nemorale) floating in the air and bec-omrae 

 detached from their parachutes and dropping to the ground whenever tliey encounter an obstacle in the course of their 

 flight. 



to close up again, and the process of dispersion is suspended until next day, when 

 the air is dry once more and the sun shining. In Andropogon Ischcemum, Avena 

 pratensis, and many other Grasses, the flowering glume has an awn composed of 

 spirally-marked and highly hygroscopic cells, and bent like an elbow, and this awn 

 undergoes a marked spiral torsion, accompanied by a slight downward flexure 

 whenever the air is dry. The distal arm of the awn is liable to get pressed against 

 objects in the course of these movements, and it then acts as a lever in raising the 

 fruits above the outer glumes. They are then easily blown away by a puff of dry 

 wind. In several Scabiouses, also, the breaking up of the fruit-capitulum, and the 

 raising of the fruits with a view to their dispersion by the wind, are occasioned by 

 a bristling movement on the part of the hygroscopic setae of the calyx. Each fruit- 

 let in the Valerian is surmounted by a pappus of delicate feathery hairs. When the 

 air is damp these feathers are folded together; when it is dry they become unfurled 

 (see fig. 471^). In this condition of divergence, they present an ample surface to 

 the wind, and the slightest gust detaches the fruits and blows them away. A similar 

 phenomenon occurs in Dry as, and in some other plants; but we cannot now enter 

 into the details of these cases. 



