FLYING-HAIRS 



545 



great distances. The form and orientation of flying-organs and their 

 relation to the mechanics of flight, cannot be discussed in detail here : 

 the organography of the organs in question lias been fully dealt with 

 by Dingier. In the present work attention must be concentrated 

 upon the anatomy and histology of organs of flight. 268 



Flying-hairs sometimes envelop the whole seed uniformly, as in 

 Gossypium. In Salix, Populus, Epilobium, Asclcpias, etc., they form a 

 terminal tuft, which acts as a parachute ; the silky or woolly pappus of 

 the fruits of Compositae illustrates this latter type of arrangement. 

 The flying apparatus of a seed or fruit usually comprises a very large 

 number of hairs ; the minute and exceedingly light seeds 

 of Acschynanthus, however, are only provided with three 

 hairs apiece. 



Typically, flying-hairs are unicellular structures of 

 considerable size ; those of Cotton, for example, vary 

 between 2 and 6 cm. in length. The multicellular 

 bristles of the Compositae are usually " feathered " in 

 various ways. The hairs which cover the rays of the 

 pappus of Tragopogon orientate, etc., become interwoven 

 into a network or web expanded in a single plane. In 

 Centaurea calocephala the two margins of the bristles are 

 fringed with hairs, which are set so closely together 

 as to form a continuous membrane, at any rate in 

 the lower half of the bristle ; by this means the surface 

 exposed to the wind is increased more than twofold 

 (Fig. 220). 



In accordance with their function, the structure of flying-hairs 

 is always such as to combine lightness with rigidity; the cell- walls 

 are at most moderately thickened, but at the same time possess very 

 considerable tensile strength. The latter property is conspicuously 

 developed in the case of Cotton, which has been tested with reference 

 to this point in the following manner. A single hair, 2 or 3 cm. in length, 

 is cemented at both ends to narrow strips of paper. One of the strips is 

 fixed immovably, while to the other is fastened a tiny paper bag into 

 which fine sand is cautiously poured ; in this way the absolute breaking 

 load of a single hair can be determined to within Tcnrth of a gram. 

 The effective cross-sectional area of the hair must then be estimated 

 by careful measurement, when the load per sq. mm. can at once be 

 calculated. By this method the breaking strain of cotton " fibres " 

 {i.e. the flying-hairs of the seeds) is found to lie between 18 and 

 22 kg.; the flying-hairs of Gossypium are therefore equal in strength to 

 the toughest bast-fibres. 



The general construction of the flying-tissues of winged seeds and 



2 m 



Fig. 220. 



Margin of a 

 pappus - bristle 



of (v ntaurea ca- 

 locephala. 



