November 



J' 



923] 



NA TURE 



657 



I 



Symbiosis in Animals and Plants.^ 



By Dr. George H. F. Nuttall, F.R.S., Quick Professor and Director of the Molteno Institute 

 for Research in Parasitology, University of Cambridge, 



(2) Root-nodtdes. 



I. Symbiosis in Plants. 



(i) Lichens. 



TT is well known to botanists that the vegetative 

 ^ body (thallus) of lichen plants consists of two 

 distinct organisms, a fungus and an alga (" gonidia "). 

 Schwendener (1867-69) regarded the fungus as living 

 parasitically upon the alga, a view which gained 

 support from subsequent researches, especially those 

 of Bonnier (1886-9), wherein synthetic cultures were 

 obtained by bringing together (a) various algae and 

 (b) fungus-spores isolated from cultures of fungi forming 

 the one component of certain lichens. 



The long and apparently healthy life of the associated 

 fungi and algae led de Bary (1879) to define the con- 

 dition as one of symbiosis, the term denoting a condition 

 of conjoint life that is more or less beneficial to the 

 associated organisms or symbionts.^ 



Investigation has shown that the relation or balance 

 between the associated organisms varies in different 

 lichens; in some the partners inflict no injury upon 

 each other ; in some, occasional parasitism of the fungus 

 upon the alga is observable. Elenkin (1902-6) and 

 Danilov (19 10) take it as proved that lichens owe their 

 origin to parasitism, the fungus either preying upon 

 the alga or living as an " endosaprophyte " upon the 

 algae that die. 



Therefore we may find in lichens the condition of 

 true symbiosis on one hand, ranging to demonstrable 

 parasitism on the other, and, conversely to what has 

 been described above, examples are known wherein 

 algae are parasitic on fungi (Beijerinck, 1890). 



The nutrition of algae in lichens is similar to that of 

 other chlorophyllaceous plants, the most important 

 work on the subject being that associated with the 

 names of Beijerinck (1890) and Artari (1902). The 

 algae associated with fungi in lichens are placed 

 advantageously in respect to nitrogen supply. The 

 important researches of Chodat (1913) have demon- 

 strated that cultivated gonidia develop four times as 

 well when supplied with glycocoll or peptone in place 

 of potassium nitrate. 



The gonidia lead a more or less saprophytic life in 

 that they obtain from the fungus-hyphae both organic 

 nitrogen and carbon in the form of glucose or galactose. 

 The nutrition of fungi in lichens depends partly upon 

 parasitism, when they invade the gonidia, and partly 

 upon saprophytism, when they utilise dead gonidia 

 (Chodat). M. and Mme. Moreau (1921) regard the 

 fungal portion as a gall-structure arising from the 

 action of the associated alga. The lichen, according 

 to this view, is to be regarded as a fungus that has been 

 attacked by a chronic disease which has become 

 generalised and necessary for the subsistence of the 

 host-fungus. 



' From llip i.i.!M.l.„li.il L.i.lrc^ ,i.liv,.i,.,l t.. Section I. (Physiology) of the 

 British » : r3. 



' J' 1 iiig the term symbiosis in re- 

 ferring ■ ^ Alien there is DO evidence what- 

 ever that ilie iaicru-i>r|{duuiui are luutually luterdependent for their growth. 



A well-known example of symbiosis is afforded by 

 the presence of the bacteroids in the nodules of 

 Leguminosae, the micro-organisms being capable of 

 fixing atmospheric nitrogen and thereby rendering 

 nitrogen available for assimilation by the plant. 

 Nodules on the roots of the alder are attributed to the 

 presence therein of Streptothrices, and comparable 

 nodules occur in Eleagnacese. The nodules on the 

 leaves of Rubiaceae and tropical Myrsinaceae are also 

 regarded as due to bacterial symbionts. 



(3) The Significance of Mycorhiza in Relation to 

 Various Plants. 



The roots of most perennial and arborescent plants 

 are invaded by the mycelium of fungi known as 

 Mycorhiza, and according to hypothesis we are here 

 dealing with symbiotic life. Frank distinguishes two 

 forms of Mycorhiza : (i) the ecto trophic, which surround 

 the root externally (found especially about the roots of 

 forest trees), and (2) the endotrophic, which penetrate 

 deeply into the root tissue and its cells. The fungus 

 utilises the reserve substances stored in the cell. The 

 intracellular mycelial mass after a time undergoes 

 degeneration, is digested by the host, and the host-cell 

 resumes its normal life. Further details regarding 

 these fungi will be found in the paper of Gallaud (1904). 



Mycorhiza in Orchids. — The first to note the presence 

 and to attempt to cultivate the fungus mycelium in 

 the roots of orchids was Reisseck (1846), and in 1881 

 Kamienski advanced the hypothesis that the association 

 was one of symbiosis. Wahrlich (1889) subsequently 

 found symbionts in all species of orchids he examined, 

 about 500 in number, thereby showing that their 

 distribution is generalised. It is to the researches of 

 Noel Bernard (1902 onward), however, that we are 

 actually indebted for the complete demonstration of the 

 true relation existing between orchids and Mycorhiza, 

 based as it is upon physiological studies. 



The essential discovery of Bernard was that orchid 

 seeds do not germinate in the absence of fungi belonging 

 to the genus Rhizoctonia. Each species of orchid, 

 according to the subsequent researches of Burgeff 

 (1909), possesses a special species, variety, or race of 

 fungus that is particularly adapted to it — he dis- 

 tinguishes fifteen species of fungus. The fungus 

 mycelium, having attained the parenchyma cells, 

 develops into characteristic filamentous masses recalling 

 the appearance seen in bacterial agglutination. After 

 a time, the development of the fungus is arrested by 

 the deeper parenchyma cells of the seeds. These 

 digest the mycelium, but the cell continues to harbour 

 remains of the fungus (" corps de ddgi^n^rescence ") 

 which occur abundantly in the tissues of orchids. 

 The seed now proceeds to sprout, giving rise to a small 

 tubercle, which at a later period produces leaves and 

 roots. The cultivation of Rhizoctonia of various 

 species was carried out successfully by Bernard, the 

 cultures being used to reproduce germination in orchids. 



NO. 2818, VOL. I 12] 



