DISPERSAL BY ANIMALS. 



829 



Here 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 adhere 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 Mamilkrias, 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.— Distribution of detached sprout like oflshoots by means of animals, 

 i Mamilldria placostigma, 2 Mamillaria gracilis. 



the animals when resting seek to rid themselves 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 oifshoots by animals is seen in 

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

 Certain species, which 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- 

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



