DISPERSAL BY WIND. 



859 



achene, and acts like a parachute, as may be seen in Oeum, Atragene, Pulsatilla, 

 aaid Olematis (see figs. 472 * and 472 0- In some Grasses, such as Stipa (see vol. i. 

 p. 619, fig. 1471), an awn is developed in the form of a long feather, which soars 

 above the tightly-closed glumes inclosing the fruit. 



Several of the fruits and seeds above described are directly exposed to the wind. 

 Owing to the fact that the desiccation of the envelopes and stalks of the fruits at 

 the time of ripening of the seeds renders certain layers of tissue brittle, a moderate 

 wind is sufficient to cause the fall of such fruits, and the same gust that brings about 

 their severance from the plant drives the fruit along 

 in a horizontal direction. The fruit does not fall to 

 the ground until the wind drops, or until its pro- 

 gress is arrested by some obstacle. 



Many other fruits and seeds detach themselves 

 spontaneously from the mother-plant when they are 

 ripe, but are not directly exposed in consequence to 

 the full shock of the wind. In these we find many 

 contrivances for the purpose of ensuring that the 

 parts to be dispersed shall be brought out from their 

 shelter, and given over to the wind at the proper 

 time. In some tropical Orchids which are epiphytic 

 on the bark of old trees (viz. Aerides, Angroecimi, 

 Sarcanthus, Saccolabium, &c.), the capsular fruits 

 contain, in addition to the small seeds, hair-like cells, 

 with spirally-marked and obliquely-pitted walls (see 

 fig. 473). Vanda teres (see fig. 475 ^) may be taken 

 as a type of this group. The hair-like cells in 

 question are woven together into a sort of felt. 

 They are extremely hygroscopic, and twist and 

 turn about in a curious manner if the slightest 

 change of condition in respect of moisture occurs. 



When the valves of the capsules move apart under the influence of a dry wind, an 

 active movement is simultaneously initiated in the matted hairs. The felt becomes to 

 a certain extent puffed up, and consequently it squeezes out between the valves of the 

 capsule, and drags the seeds, which are imbedded amongst the hairs, from the interior 

 to the surface of the capsule, where they are liable to be blown away by ,the least 

 breath of wind. This happens, as was said, when a dry wind is blowing. In wet 

 weather the capsules close up, and conceal both hairs and seed once more in their 

 interior. Similar phenomena may be observed in the fruit-capitula of some Com- 

 posites whose fruits are spontaneously detached from the receptacle on ripening. 

 In damp weather the loose achenes lie hidden in the involucral cup, as though at 

 the bottom of a basket, and the hairy pappuses appended to the achenes are clubbed 

 together. When the atmosphere is dry, the involucre, which is composed of hygro- 

 scopic scales, opens, and the pappuses of the fruits within spring apart, and so act as 



rig. 473.— Seeds of the Orchid Vanda teres, 

 which are moved from the interior to the 

 surface of the capsule by hygroscopic 

 hair-like cells, and are thus exposed to 

 the wind; xlOO. 



