243 



METABOLISM 



ficial cells of the root, but these are always confined to the intercellular spaces. 

 This association is accompanied by certain differences in form and anatomical 

 structure in the root, which render the presence of the mycorhiza easily recogniz- 

 able, and which must have a physiological significance. One of these 

 differences is the non-formation of roothairs on roots provided with fungus 

 mycelium (Fig. 45), so that the absorption of all nutrients and water is possible 

 only through the medium of the fungus. From what has been already said 

 as to the small amount of nitrogen in woodland soils (p. 133), one can only 

 conclude that the Fungi but not the trees are restricted to ammonia ; but 

 we are ignorant what the nitrogen compounds are which the root receives 

 from the fungus covering. STAHL'S hypothesis would seem at first sight to 

 be much more applicable to this type of mycorhiza than to the endotrophic 

 form. On further reflection, however, difficulties appear here also. If the 

 fungus actually anticipates the tree in acquiring mineral materials from the 

 soil, why does it not retain them ? Why does it, after assimilating them, give 

 them up again ? In the endotrophic mycorhiza this is explained by the fungus 



being digested, but there is no 

 reason to believe that a similar 

 process takes place in the ecto- 

 trophic type. Several problems 

 await solution here, and it is 

 astonishing how little experi- 

 ment teaches us in this re- 

 lation. And yet systematic 

 experiments are not to be con- 

 sidered as hopeless ; we know 

 how NOBBE (1899) managed to 

 develop pines, firs, larches and 

 c \s^f beeches with perfect success for 

 a period of twenty-five years 

 in pure quartz sand, free from 

 humus. At all events, there 

 appeared to be no hereditary 

 fixing of the adaptation, and 

 hence experiments are not hopeless. According to HOLLER'S (1902) researches, 

 the significance of ectotrophic mycorhiza has again been called in question 

 [since the Brandenburg pine has all the less mycorhiza the more humus the soil 

 possesses]. It is to be expected, however, that these experiments will show 

 that among mycotrophic plants, in addition to real cases of symbiosis, there 

 are associations which are harmless and perhaps of no importance at all, 

 and others which are really cases of parasitism; it may be that the fungus 

 is the parasite, and again it may be that the seed-plant is the parasite. In 

 many endotrophic mycorhiza on green plants the fungus must be the parasite, 

 among the ectotrophic types on colourless plants (e. g. Monotropa) it must be 

 the seed-plant that is parasitic. 



In speaking of symbiosis, it is impossible to ignore the case of lichens, 

 to whose very remarkable association of fungus and alga DE BARY (1879) 

 first applied the term symbiosis. The conception of a lichen as a combination 

 of alga and fungus types was first enunciated by SCHWENDENER (1869, Algen- 

 typen der Flechtengonidien. Basel). On this question DE BARY (1865, Morph. 

 u. Phys. d. Pilze, Flech ten, &c.), and also REINKE (1894, Jahr.f.wiss. Bot. 26,524) 

 should be consulted. Notwithstanding the work which has been accomplished, 

 we do not even yet completely understand the modus vivendi of this symbiosis. 

 BEIJERINCK (1890) and ARTARI (1899) have shown that certain lichen-Algae 

 are ' peptone plants ', and we may suppose that the fungus contributes peptone 

 to the combination, while it is naturally the business of the alga to assimilate 



Fig. 45. /, Beech root from woodland humus (magnified) ; /, 

 fungal cords, united with soil particles at a. If, Beech root from 

 sterile humus (magnified) ; c, rootcap ; A, roothairs. (From 

 DETMER'S Smaller Practical Botany.) 



