DISPERSAL BY ANIMALS. 



829 



Hei« some of the spherical, closely-crowded lateral shoots growing from the old 

 plant loosen spontaneously and fall to the ground; others again remain in situ 

 but adiiere very slightly, so that a passing contact or a gentle touch is enough to 

 complete the separation from the old plant. Now bristles are formed at the top 

 of each papilla of these Mammillarias, some of which end in barbs, so that the 

 spherical shoots resemble burs. They adhere just like burs to the hairy paws or 

 fur of grazing animals, which carry them away unconsciously. Afterwards 



Fig. 457. — Distribiu. i ii-.il sj.n.ut In.- n-.njs of animals. 



1 ilammillaria pJacostigma. 3 Mammillaria gracilis. 



the animals when resting seek to rid tliemselves of the inconvenient appendages, 

 brushing them off and leaving them behind on the ground. Here they may strike 

 root and grow up into new plants. 



The third method of distribution of sprout-like offshoots by animals is seen in 

 aquatic plants, wliich fasten either entirely or in fragments to passing water-birds. 

 Certivin species, wliicli very rarely blossom or form fruit, but nevertheless occur in 

 innumerable widely distant spots and often appear unexpectedly in newly-formed 

 ponds, in artificial lakes, and in other waters, are for the most part distributed by 

 water-birds. Some of these water-plants, e.g. the Frogbit and Bladderwort {Hydro- 

 diaris and Utricularia), develop peculiar slimy coverings round their buds, which 



