MESSMATES, MUTUALISTS, AND PARASITES 95 



and slugs, are not kept off by smooth surfaces or sticky secretions. 

 But such creatures are easily baffled by prickles, bristles, thorns, 

 and other sharp structures, and these are often found in the neigh- 

 bourhood of the flowers. 



DISPERSAL OF PLANTS BY ANIMALS. Since the large majority 

 of plants are fixed, means of dispersal are clearly a necessity, as 

 otherwise they would have to struggle for existence with their 

 own offspring. And it is only when numerous individuals of a 

 species are placed in favourable surroundings that the species 

 has any chance of escaping extinction. It is not therefore surpris- 

 ing to find that there are almost innumerable ways by which dis- 

 persal is effected. Sometimes the plant itself is the agent, sending 

 out creeping stems above or below ground, or ejecting its fruits, 

 seeds, or spores to a distance by explosive or elastic mechanisms. 

 Currents of air and water are also of great importance in this 

 connection. But we are here only concerned with the chief ways 

 in which animals are pressed into the service of plants for this 

 purpose, or it may be render assistance of more casual kind. 



Many of the small plants which float in ponds, such as Duck- 

 weeds (Lemna) and various algae, must often cling to the legs of 

 water-birds, and get carried bodily from place to place. And it 

 is noticeable that the buds of somewhat larger aquatic plants, 

 such as Frog-bit and Bladderwort (Hydrocharis and Utricularia), 

 possess a slimy covering by means of which they readily adhere 

 to the plumage of such birds. Among marine plants a curious 

 means of transit is exemplified by various sea-weeds which certain 

 crabs plant on their backs to make themselves inconspicuous (see 

 vol. ii, p. 287). On the decease of such a crab his little ''garden" 

 goes on growing, unless perchance he has been swallowed whole 

 by some predaceous form. 



A good many land-plants propagate by means of " offshoots ", 

 i.e. specialized branches, &c., which grow into new individuals, 

 and cases have been noted where animals assist in the dispersal 

 of such offshoots. Some of the rounded Mexican Cacti (species 

 of Mammillarid], for example, produce little spherical shoots 

 studded with barbed bristles, and which are very readily detached 

 from the parent plant. They readily cling to the coats of various 

 mammals and may thus be carried for a considerable distance. 



Dispersal of Seeds and Fruits by Animals. As already 

 explained (p. 85), a seed may be regarded as a matured ovule, 



