SYMBIOSIS AND "VEGETATING ANIMALS:' 817 



and in what conies hereafter we must to a great extent follow his lead. 

 The algae which are found in animal substance have been referred to 

 above as " parasitic," but it is chiefly to avoid the use of this term that 

 the more accurate one (symbiosis) has been employed. A closer anal- 

 ogy than that offered by the lichens would be, it seems to me, afforded 

 by any perfect plant an oak, for example. Here the colorless cells 

 of the root, let us say are bound to live at the expense of the green 

 cells in the stem and leaves. Yet we do not think of this as a para- 

 sitic event. The root-cell is rather a unit in a vast colony of units 

 (cells) associated for mutual benefit. The green cell gets quite as 

 much good from the root-cell as the latter gets from the green cell ; 

 water and salts are exchanged by the root-cells for sugary matters and 

 other things readily made use of by any cell, and no harm (as would 

 be the case in parasitism), but rather much good, is done by the ex- 

 change. Evidently the oxygen thrown off by the alga is precisely 

 what the plant needs, and the carbonic acid and nitrogenous waste 

 eliminated by the animal is most useful to the alga. Moreover, the 

 algae gain the advantage of ready locomotion with their host, and the 

 animal can go further into unfavorable media when stocked with algae 

 ready to build up starches and sugary matters from carbonic acid and 

 water. If either dies, the other is the gainer ; since the algae can thrive 

 on the pi'oducts of animal decomposition, and algae digestible, i. e., 

 dead algae are much esteemed by most animals. 



The whole burden of the physiological history of symbiosis is for- 

 cibly summed up by Mr. Geddes as follows : " Thus, then, for a vege- 

 table cell no more ideal existence can be imagined than that within the 

 body of an animal cell of sufficient vital activity to manure it with car- 

 bonic acid and nitrogen waste, yet of sufficient transparency to allow 

 the free entrance of the necessary light. And, conversely, for an ani- 

 mal cell there can be no more ideal existence than to contain a vege- 

 table cell, constantly removing its waste products, supplying it with 

 oxygen and starch, and being digestible after death. ... In short, 

 we have here the relation of the animal and the vegetable world re- 

 duced to the simplest and closest conceivable form. 



" It must be by this time sufficiently obvious that this remarkable 

 association of plant and animal is by no means to be termed a case 

 of parasitism. If so, the animals so infested would be weakened, 

 whereas their exceptional success in the struggle for existence is evi- 

 dent. Anthea cereus, which contains most algae, probably far out- 

 numbers all the other species of sea-anemones put together, and the 

 radiolarians, which contain yellow cells, are far more abundant than 

 those which are destitute of them. . . . Such an association is far 

 more complex than that of the fungus and alga in the lichen, and 

 indeed stands unique in physiology as the highest development, not 

 of parasitism, but of the reciprocity between the animal and vegetable 

 kingdoms." 



vol. xxiii. 52 



