1564 



A TEXTBOOK OF THEORETICAL BOTANY 



hooked, and the fruits he on the ground always with one spine erect. 

 This pierces the feet of animals or gets between their toes, or it may be 

 picked up by rubber tyres. In all these ways it gets transported, though 

 it often causes great suffering to the animals, for which reason it is called 

 the Devil Thorn in South Africa. Such a method recalls Tribiihis, a mem- 

 ber of Zygophyllaceae, in which each of the five carpels of the fruit bears 

 two divergent spines which stick out all round so that some of them are 

 erect however the fruit lies. They also pierce the feet of animals and cause 

 much trouble through being thus carried off. In this case the spines are 

 direct outgrowths of the pericarp, not of the sepals. (See also below, 

 p. 1565, under Pedaliaceae.) 



An instance where spiny bracts assist in dispersal is given by the genus 

 Arctium, the Burdock, in which each member of the involucre surrounding 

 the capitulum becomes hardened and ends in a hook. Their efficiency in 

 promoting distribution needs no remark to anyone who knows the English 

 countryside. A curious example of the use of a stem structure as a means 

 of adherence is that of Vncinia, which is a southern hemisphere relative of 

 Carex, with only one species {U. tnicroglochin) in the north. The rachilla 

 of the spikelet, which is abortive in Carex, is here prolonged into a sterile 

 outgrowth, ending in a hook, which protrudes from the perigynium and 

 very readily catches in fur, feathers or wool. 



Many grass fruits are dispersed by animals to which they adhere by 

 means of hairs on the glumes or of barbs on the awns or simply by the 

 penetration of the awns into clothing. The number of Grasses which can 

 be transported in this way is familiar to every country rambler, in fact 

 most of the common grasses are concerned in it, especially Bromus sterilis, 

 Bromus racemosus, Festuca, Holcus, Hordeum, Poa, Helictotrichon (Avena), 



Alopecurus, Trisetiim and Arrhena- 

 theriim. In other parts of the world 

 the dispersal of grasses in this way 

 is equally common and is shown by 

 such widespread genera as Tragus, 

 Eriochloa, Cenchriis, Thenieda, Stipa, 

 Oplismenus and many others. One 

 that requires a special mention is 

 Heteropogon contortus, a well-known 

 pest of grazing animals in many 

 parts of the world (Fig. 1425). The 

 spikelet bears an awn several inches 

 long, the lower part spirally twisted, 

 as in some species of Stipa. At the 

 base is a tuft of sharp, stiff, upright 

 hairs. The spiral portion twists and 

 untwists hygroscopically and drives 

 the sharp-pointed base, with its 

 grapnel hairs, into the wool and 



/ 



^^^ 



Fig. 1425.- — Heteropogon contortus. Mass of 

 the long-awned fruits which have twisted 

 themselves together. 



