254 ANIMALS AND PLANTS A SYMBIOTIC COMMUNITY 



pretation possible in this case, for all the other hypotheses, — such as that Monotropa 

 enters into connection with the roots of trees, or that it is parasitic in the first 

 stages of development, but subsequently detaches itself from its host and becomes a 

 saprophyte, — rest on inaccurate observations, and have long been disproved. As a 

 parasite Monotropa ought to have been discussed at the same time as others in 

 earlier pages, but it was not without intention that the description of this plant was 

 reserved for this place, for it would have been difficult to state and explain the 

 method of nutrition exhibited by it before some previous knowledge of the curious 

 phenomena of union of the mycelia of fungi with the roots of green-leaved 

 Phanerogams had been acquired. 



ANIMALS AND PLANTS CONSIDERED AS A GREAT SYMBIOTIC 



COMMUNITY. 



If we look back at the cases of symbiosis already discussed and inquire what is 

 their value, we find it consists in an integration of the functions of plants possessing 

 chlorophyll and plants not possessing it. The reciprocity here implied is, however, 

 at bottom, but a copy of the complementary interaction of plants and animals which 

 takes place on a grand scale in the organic world. The associated plant, destitute 

 of chlorophyll, in which capacity fungi are always the organisms concerned, really 

 plays the same part in the social life as is taken by animals in the great economy 

 of nature, and this is in harmony with the fact that in other respects as well 

 fungi exhibit so many similarities to animals that in many instances one looks in 

 vain for a line of division to separate them from animal organisms. Hence there is 

 no need for surprise when cases come under observation wherein a quite unmis- 

 takably animal organism enters, instead of a fungus, as one of the partners in a 

 symbiotic community. Certain Radiolarise have small yellowish spots upon them, 

 which were formerly held to be pigment-cells, but have pt'oved to be little alga?, 

 with cells furnished with true chlorophyll. Similar properties are exhibited by 

 the fresh-water polyp, Hydra, and by the marine sea-anemones. Small alga occur 

 in social union with these also in the shape of cells with membranes made of cellu- 

 lose and containing chlorophyll and starch-grains in their protoplasmic bodies. 

 These algae are in no wise injurious to the animals with which they are associated; 

 on the contrary, their presence is beneficial, their partners reaping an advantage 

 from the fact that the green constituents split up carbonic acid under the influence 

 of the sun's rays, and in so doing liberate oxygen which may be again taken in by 

 the animals direct, and serve a useful purpose in their respiration and all the pro- 

 cesses connected therewith. Conversely, the alga, in association with the animal's 

 Vjody, will derive a further advantage from the latter, inasmuch as it receives at 

 first hand the carbonic acid exhaled by the animal in breathing. The small algaj 

 living socially with animals cannot be reckoned as parasites in any case, nor 

 can the animals be looked upon as parasites of the algae, but we have heir the 

 phenomenon of mutual assistance and of a bond serving for the benefit of both 



