328 PLANT-LIFE 



bottom of the water. Mountain streams may transport 

 seeds of Alpine species to levels in the lowlands. The 

 waters of the Gulf Stream have frequently borne fruits 

 and seeds of Mexican plants to the shores of the North 

 Cape. To secure the fullest advantage of water trans- 

 port, seeds and fruits must be able to float, and also to 

 resist damage by the water itself. The fibrous covering 

 of the Cocoa-nut is so woody and thick that sea-water 

 cannot penetrate to the nut. But transport by water 

 has great limitations ; it can bear seeds only to the banks 

 of streams and sea-coasts. 



Wind is a more efficient and satisfactory transport 

 agency than water. We have seen how well it serves 

 the Sycamore (p. 326), and also how the fruits of that 

 tree are adapted to wind-transport. The fruits of the 

 Ash, Elm, and Maple are of a similar character. There 

 is a plant of New Granada, called the Bignonia {Bignonia 

 echinata), whose seeds are furnished with very delicate 

 membranous wings, which act like the wings of an 

 aeroplane, and enable the seed to describe hawklike 

 circles in the air. A large bract occurs at the base of the 

 inflorescence of the Lime-tree {Tilia). When the fruit 

 is ripe the complete inflorescence becomes detached 

 from the tree, and falls with a rotary motion head down- 

 wards; it is the bract which induces the motion, and, 

 of course, ensures a delay in descent during which the 

 wind may carry the fruit for some distance. A large 

 number of fruits and seeds have hairy appendages, which 

 enable them to be caught up by the wind. This is 

 specially the case among the highly specialized Com- 

 positse (p. 216), in which, with some exceptions, the 

 calyx of the individual floret has been modified into a 



