ANIMALS AND PLANTS A SYMBIOTIC COMMUNITY. 255 



parties, precisely similar to that noticed in the case of lichens and in the others 

 which have been described above. 



Several of the liverworts which live as epiphytes on the bark of trees exhibit 

 on the under surface of their leaflets (which are inserted on the stem in two rows, 

 and are pressed flat against the bark) little auricular structures, and in species of 

 the genus Frullania, these take the form of definite hoods or pitchers. The rain 

 that trickles down the trunks of the trees, washing the bark and wetting the liver- 

 worts in its course, fills the hooded receptacles referred to with water, and is retained 

 longer in these protected cavities than anywhere else, if a period of drought ensues 

 and the liverwort becomes dry again. Now these cowls are the abode of tiny 

 rotifers (Callidina symbiotica and C. Leitgebii), which live on the organic dust 

 brought thither with the water. In return for the peaceful home thus afforded 

 them in the hooded chambers of the leaves, the rotifers supply the liverworts in 

 question with nitrogenous food. For as such must serve the matter excreted by the 

 rotifers in the interior of the cowls. Without the intervention of the rotifers, the 

 living organisms (Infusoria, Nostocinese, and spores) contained in the water could 

 not be converted into food by the liverworts, whereas the liquid manure arising 

 from the Infusoria, Nostocinese, and spores, digested in the bodies of the rotifers, 

 contains highly nitrogenous compounds, which are of great value to the liverworts 

 in question, as indeed they are to all epiphytes living on the bark of trees. It 

 stands to reason that the symbiotic liverworts and rotifers derive also a mutual 

 advantage from the fact that the oxygen set free by the former comes into the 

 possession of the rotifers and the carbonic acid emitted by the rotifers into that of 

 the liverworts by the most direct method. 



Moreover, these cases of partnerships further remind us of other analogous rela- 

 tions existing between plants and animals, which it is necessary to refer to now, 

 although they cannot be treated in detail till later on. A great number of flowering- 

 plants excrete honey into their flowers, and so attract flying insects to them, 

 which supply themselves plentifully, and in their turn render to the plants they 

 visit the service of transferring the pollen from flower to flower, thus making 

 possible the development of fruits and fertile seeds. Certain small moths which 

 visit the flowers of Yucca bring the pollen to the stigmas, and force it into the 

 stigmatic orifices in order that mature fruits and seeds may be produced from the 

 rudimentary fruits, a result which is indeed a matter of vital importance to these 

 moths. For the moths lay their eggs in the carpels of Yucca, and from the eggs 

 larvae are developed which live exclusively on the seeds of this plant. If the 

 Yucca were not fertilized, and did not develop any fruit, the larvae would die of 

 hunger. A similar phenomenon occurs in many other cases of the kind, where 

 both plant and animal reap some benefit. On the other hand, in the formation of 

 galls, which are produced by animals laying their eggs in particular parts of plants, 

 the advantage (with few exceptions) is all on the side of the animals, and these gall- 

 structures might most justly be placed by the side of parasitic structures. 



It is obvious from all this that such of the mutual relations of plants and of 



