196 LIFE : OUTLINES OF GENERAL BIOLOGY 



in long lines, which float away with a beautiful wavy motion, like 

 silver seq^ents in the air. The dispersal of the linked fruits may be 

 followed for m;my yards; but gradually the links are broken. Even 

 then, each feathered achene continues to be wafted until it for- 

 tuitously sinks to earth and is more or less fortuitously moored. 

 Unfortunately for nKui, many "weeds" are eftectively spread by this 

 parachuting adaptation. 



In some cases the parachute takes the form of a light wing-like 

 exjxinsion, as in the maple, and this brings about a whirling oblique 

 movement when the fruit is wrenched off by the wind. By its gyra- 

 tions the fruit is borne beyond the shaded zone around the parent 

 tree. The frequency of short distance parachuting suggests that it 

 may be as imjx)rtimt as a long journey; but perhaps its frequency 

 merely indicates that it is of course more readily attained. It is 

 particularly common in trees, such as maple, ash, and elm. From the 

 ecological jx^int of view it is almost immaterial whether the para- 

 chuting apparatus is carried by the fruit or by the seed', both 

 arriuigements are common. Hairs are attached to the seeds of the 

 cotton-plant (Gossj'pium), but to the fruits of the cotton-grass 

 (Eriophoron) ; a wing is attached to the seed of the pine, but to the 

 fruit of the sycamore (maple), like our common British tree mis- 

 named "plane", so different in fruiting from the true plane of London 

 avenues. 



An unusual mode of dispersal is illustrated by some desert plants 

 which break oft from their roots when mature and are driven by the 

 wind along the ground, liberating their seeds as they go. Such are 

 the American Tumblewceds and Pigweeds, which illustrate in an 

 interesting way how a desiccation natural to the habitat may be 

 turned to an advantage in dispersal. The Glasswort {Salsola kali), 

 one of the Goosefoot order, Chenopodiacca?, but often called the 

 Russian Thistle, shows the same habit of detachment, and has of 

 recent years become a serious agricultural pest in North America. 

 In the Rose of Jericho {Anastatica hierochuniina), a Crucifer of the 

 Mediterranean region, the leaves fall off in the dr>' season, when the 

 studs arc ripening, the branches fold together, and the whole plant 

 breaks off. It is driven about like a light ball by the wind, but the 

 po<is remain closed until it reaches moisture or the rains return. Here 

 may be mentioned the fruit of the Porcupine Grass (Stipa), which 

 has a long spirally twisted a\vn and at its base a sharp spine. The 

 awn is wiy hygroscopic, untwisting in moisture, coiling again in 

 drouglit; and these changes enable the fruit to creep on the surface 

 of the ground and to bore into a minute crevice. The beginning of 

 this is seen in some other grasses and in the "stork's bill" of many 

 wild geraniums. 



(2) Many truly aquatic plants, such as pond-weeds and water- 

 lilies, have their fruits or seeds normally distributed by currents. 



