DISPERSAL BY WIND. 861 



In the case of Willow-herbs (Epilobium) and of some Pines (Pinus nigricans, 

 P. sylvestris, &c.) the fruit-valves and fruit-scales which cover the seeds only open 

 back under the influence of the sun's warmth, and when a dry wind is blowing, 

 and the same wind which thus operates on the valves and scales also carries off the 

 seeds the moment they are exposed, they being furnished with wings or tufts of 

 hair with a view to aerial dispersion. The reader must be referred to p. 447 for a 

 description of the manifold effects of a dry wind on the fruits and seeds in question. 

 First, the dry capsules open; secondly, the seeds, hitherto lying in the interior of 

 the fruits, where they are protected against moisture, are shaken out by the swayino- 

 to and fro of the elastic fruit-stalks; and thirdly, these seeds are caught up and 

 scattered by the wind. 



The distance to which fruits and seeds which are adapted to aerial transit by 

 means of wings, hairy tails, parachutes, inflated envelopes or woolly coverings, as the 

 case may be, are conveyed by the wind depends on the degree of perfection of their 

 mechanism, on the condition of the air in respect of moisture, and on the strength 

 of the current of air by which they are transported. When the weather is calm and 

 sunny, innumerable of the lighter fruits and seeds are carried up to a great height 

 by the ascending currents which are generated in the atmosphere; but they usually 

 descend again after sunset at a little distance from the spot where they were taken 

 up. Such excursions do not conduce so much to a dispersion of plants over large 

 areas as to their deposition on shelves and in crevices of steep walls of rock, where 

 seeds would not otherwise easily acquire a footing. Currents moving in a horizontal 

 direction may, it is true, convey their freight of fruits and seeds over extensive 

 tracts of country, but very exaggerated notions are usually entertained concerning 

 the distances thus attained. Amongst the numerous species of fruits and seeds 

 blown by storms of wind to the tops of the Alps and left upon the snowfields above 

 the glaciers, not a single one derived from distant parts (i.e. from another district) 

 has been found after careful examination of the deposited matter; and from this 

 we may infer that, even on mountains, fruits and seeds are scarcely conveyed any 

 further by a raging wind than when they are blown from one side of a valley to 

 the other. 



In many plants the wings or parachutes, as the case may be, only remain 

 attached to the seeds or fruits for the period of their journey through the air. If 

 the winged seed of a Pine gets stranded anywhere the membranous wing drops oft', 

 and the seed is then no longer capable of flight. This phenomenon is even more 

 marked in the fruits of Thistles (e.g. Carduus and Cirsium; see fig. 474). The 

 achenes, which are comparatively large, are supported by parachutes and float 

 quietly in the air, but the moment one of them strikes against any obstacle the 

 fruit severs itself from the parachute and falls to the ground. There can be little 

 doubt that to this mode of dispersion must be attributed the common occurrence of 

 Thistles at the foot of walls and in hedgerows, inasmuch as the floating fruits are 

 carried against such structures with especial frequency. In other cases the fruit or 

 seed maintains permanently a firm connection with the parachute, and the latter 



