THE ANGIOSPERMAE 1561 



resemble centipedes and may be picked up in error by foraging birds 

 (Fig. 1422). Direct proof of this is, however, lacking. 



Further examples of this apparent mimicry may be seen in the inde- 



FiG. 1422. — Bisenula pelecynits. Legumes 

 mimicking centipedes. 



hiscent legumes of Scorpiiirus, of which S. siibriUosa resembles a centipede 

 and S. vermiculata a caterpillar. The seeds of Abriis precatorhis and of 

 Jatropha spp. have also a rather striking likeness to beetles. 



Attachment externally to animals is usually ensured by prickly or 

 hooked hairs or by stickiness. Hairs and bristles may be a means of pro- 

 tection rather than of dispersal, especially in the unripe condition of the 

 fruit, when it is desirable that the seeds should be protected from distur- 

 bance. Bristles on the outside of dehiscent capsules serve such an end, for 

 when they dehisce the seeds are ripe and are then fully exposed. The 

 fruits of many Mimosaceae, likewise, are spiny around the septum, but 

 the unprotected valves drop or are picked out of this prickly framework 

 when they are ripe. The same prickles which protect the unripe fruit may, 

 however, assist its dispersal when ripe, as in some species of Medicago 

 {M. tiirhinata, M. arahica), where the margin of the spirally curved legume 

 is furnished with two rows of crossed prickles which easily catch in a 

 woolly covering. 



The prickles are commonly hardened hairs, often with hooked tips, as 

 in Galium aparine and Myosotis arvensis. They may be produced on the 

 pericarp, as in the former example, or on the persistent calyx as in the 

 latter, the enclosed fruit being smooth. In Agrimonia the prickles are 

 formed on the outside of the perigynous receptacle which encloses the 

 two ripe akenes like a calyx. The list of other plants whose fruits are 

 attached to animals or to clothes by such hooked hairs is a long one, but 

 examples need not be multiplied beyond a few common genera: Daucus, 

 Caucalis, Xanthium, Urtica, Cynoglossum and Circaea (Fig. 1423). 



Hooked or viscid hairs are not always confined to the fruit parts but 

 may be formed on leaves or branches as well and in some plants whole 



