102 INTRODUCTION TO PLANT GEOGRAPHY [CHAP. 



family (Bignoniaceae), and by Pines and Spruces and many other 

 Conifers. 



(/) Winged fruits. Again chiefly occurring on trees and shrubs, 

 these are so modified as to cause the fruit, on detachment by the 

 wind, to be borne at least out of the immediate sphere of influence 

 of the parent plant, or to trundle along as is the case with many 

 bladder-fruits. Often the flight is a spinning one and, though 

 spectacular, not very efficient in terms of distance. Each fruit (as 

 n the Birches, Betula spp.), or half of a separating fruit (as in most 

 Maples, Acer spp.), is usually one-seeded — functionally, at least. 



[g) Long-haired seeds and fruits. These are sufficiently alike to 

 be considered together, while also approaching (c) and [d), their 

 main feature being that the surface is covered with long silky or 

 woolly hairs. Such disseminules tend to be less efficient than 

 plumed ones but are nevertheless capable of travelling for some 

 miles. The plants, as in categories {e) and (/), are most commonly 

 trees or shrubs. Examples of seeds of this nature are those of 

 Cotton (Gossypium), Willows {Salix spp.) and Poplars {Popuhis 

 spp.) ; and of fruits, those of some Anemones {Anemone spp.). 

 That this mode of dispersal is abundantly eifective, at least so far as 

 transport of the seeds is concerned, has been frequently and 

 strikingly demonstrated to the writer when he has looked out from 

 his laboratory windows in the ancient Botanic Garden at Oxford 

 and thought a snow-storm was raging, the ' flakes ' being masses of 

 hairy seeds blown from pollarded Willows mostly hundreds of yards 

 away. 



(h) Tumble-weeds. Such plants, or detached portions bearing 

 the seeds, tend to roll before the wind or be blown across open 

 country, usually scattering their seeds or fruits as they go. They 

 are commonly short-lived herbs that branch densely and stiffly from 

 a central stem and have a rounded form. Normally they break off 

 easily near ground level and have the seeds or fruits so loose that 

 they are lost as the aerial part trundles along. Tumble-weeds occur 

 chiefly in deserts or arid prairies or steppes. Examples include the 

 so-called Russian-thistle {Salsola pestifer) in North America and 

 Eryngium sp. on the northern border of the Sahara in Egypt (R. W. 

 Haines voce). In less ideally displayed form, all manner of plants 

 or parts of plants can, in special circumstances, act fortuitously as 

 tumble-weeds — including the so-called Rose of Jericho {Anastatica 

 hierochuntind), and some Lichens and Mosses in the Arctic. 



(?) Other organs or methods. These include pieces of such 



