1560 A TEXTBOOK OF THEORETICAL BOTANY 



which seeds and fruits will float. Their results are not always consistent 

 and it is evident that there must be a good deal of natural variation in this 

 respect and that probably a great many fruits sink quickly and are lost. 



Dispersal by sea is not by any means confined to trees. Seashore plants 

 in all parts of the world may be carried by sea currents to fresh shores. 

 Among north temperate species we may mention especially: Crambe 

 maritima, Cakile maritima, Lathyrus maritimiis, Honckenya peploides, 

 Cnthtmim maritimum, Calystegia soldanella, Euphorbia paralias, and Spar- 

 tina townsendii. Common seashore species of warm climates which are 

 dispersed by sea include the following, some of which are trees or shrubs 

 and some herbaceous: CalophyUmn inophyllum. Hibiscus tiliaceus, Carapa 

 moluccensis, Canavalia rosea, Erythrina, Mucuna, Entada and Cassia 

 (various species), Terminalia catappa, Barringtonia racernosa, Heritiera 

 littoralis, Pemphis acidula, Sesuvium portulacastrum, Selliera radicans, 

 Scaevola koenigii, Ipomoea pes-caprae, Pandanus fascicularis, Remirea mari- 

 tima, Fimbristylis spathacea and Thuarea sarmentosa. This list includes 

 only the species with wide distributions and excludes most of the Mangrove 

 species (which we have mentioned before and will deal with again later) 

 and a few which have been specially mentioned above. In some of these 

 species it is the fruit which floats and in some it is the seed, but we have 

 thought it best to keep them together as they form a natural biological 

 group. 



The last-named species, Thuarea sarmetitosa, deserves a word of 

 special description for its peculiarity. It is a grass of sea sands, with a 

 short spike of flowers borne on a very broad rachis. The upper flowers 

 are male and the one female spikelet is at the base of the rachis. The latter 

 folds over the female spikelet as the seed ripens and then becomes thick 

 and hard, forming an excellently protected floating body. It is principally 

 found on islands in the Indian and Pacific oceans. 



Animal Dispersal. The dispersal of fruits by animals is chiefly external. 

 When fruits are eaten by birds or animals it is usually the seeds which 

 are scattered or excreted and we have already referred to this method of 

 dispersal in dealing with seeds. An exception to this statement must be 

 made in the case of small, hard fruits, mostly of herbaceous plants, which 

 are regularly swallowed by cattle in large numbers while browsing and are 

 excreted, like many seeds, unharmed. Among common genera thus dis- 

 persed are: Ranunculus, Galium, Leontodon, Urtica, Atriplex, Chenopodium, 

 Polygonum, Rumex and of course the caryopses of many pasture Grasses. 

 For some of these genera such a mode of dispersal is of primary importance, 

 for oXhe-x?,, e.g., Leontodon and Galium, it is only secondary to dispersal by 

 other means. 



It has frequently been suggested that certain seeds, like those of Ricinus, 

 are dispersed by birds because of their resemblance to beetles, which 

 would attract insectivorous birds. We have already seen that there is 

 reason to doubt this, but another case of mimicry which may have an 

 analogous effect is that of the fruit of Biserrula pelecynus, whose legumes 



