140 COACTION: THE INTERRELATIONS OF ORGANISMS 



From their life form, lichens also exhibit attachment coactions, as a 

 result of which they may injure bark or leaves by shading, and in 

 the higher forms, the thallus may also live parasitically on the bark, 

 as Phillips has shown for Usnea (1931, d). 



The association of fungal hyphae, regularly of Hymenomycetes, 

 with roots has long been known as mycorhiza and explained as a rule 

 in terms of mutualism. Its exact nature is still much debated, how- 

 ever, and recent investigators are in disagreement as to the proper 

 interpretation of the majority of cases. This is readily intelligible in 

 the light of Melin's statement that a double parasitism is involved 

 (1925). In the case of conifers, he states that the fungus supplies 

 organic nitrogen to the holophyte, perhaps also potassium and phos- 

 phorus, and in extreme cases serves to absorb all the water and nutri- 

 ents needed by the host. On the other hand, McDougall regards all 

 ectotrophic or external mycorhizas of forest trees as pure parasites 

 (1914, 1922), and Masui (1927) states that it is going too far to say 

 that mycorhiza is in general a symbiotic phenomenon, but that on the 

 contrary it is purely parasitic, or at most only hemi-symbiotic. To 

 the latter he assigns the mycorhizas formed by Boletus, to the former 

 those produced by Armillaria, Polyporus, Hydnum, etc. 



Mycorhizas are widely distributed through the orders of seed 

 plants, and are also found in some ferns and liverworts. However, 

 they are more characteristic of the roots of conifers, of diclinic trees 

 such as alder, beech, birch, oak, etc., of Ericaceae, Rutaceae, and 

 Orchidaceae. The fungus symbionts belong chiefly to the gill fungi, 

 with some members of the pore fungi and puffballs, and rarely of 

 other groups. Practically all the genera concerned also grow sapro- 

 phytically in the soil and thus exemplify another type of coaction 

 bond in the community. 



Plant and Animal. The major categories of this type are coac- 

 tions (1) between invertebrates and algae, rarely bacteria or fungi; 

 (2) between insects and fungi; and (3) between flowers and pollinators. 

 The animal symbionts with green algae, such as Zoochlorella, Chlam- 

 ydomonas, Pleurococcus, and Scenedesmus, are represented by infu- 

 soria (Amoeba, Frontonia, Paramecium, Stentor, Vorticella, etc.), a 

 few sponges, hydroids, ophiurids, and the flatworm Convoluta. These 

 coactions have been much studied, and in the Infusoria in particular 

 prove to be comparable with that in the lichens, the alga as a species 

 deriving some benefit but the individual succumbing to parasitism. 

 An identical coaction is exhibited by a yellow algal symbiont, Zooxan- 

 thella, with Foraminifera, Radiolaria, ciliates, sponges, and Bryozoa, 

 and Convoluta as well. From their very structure, sponges are also 



