308 STUDIES IN SYMBIOSIS. 1., 



The next problem to be solved is why does the fungus enter the root of this 

 saprojjhyte? The mycelium of the fungus occurs in the soil and in tangles on 

 the surface of the root; hyphae from these tangles enter as far as the cortex of 

 the root. This penetration is probably the result of chemotropism ; the hyphae 

 penetrate more deeply into the root tissues because of the directive attraction of 

 substances in the cells. These substances must be present in greater quantity in 

 the root than in the soil. The substance concerned in this case is probably sugar 

 (or starch), which I have already indicated is present in the host-oells and dis- 

 appears after infection. The starch present cannot be the product of photo- 

 synthesis; it is probably manufactured by the host-cells from carbonaceous 

 substances absorbed from the humus by the mycelium of the fungus in some 

 other infected part of the root. The starch (or sugar) could be formed in 

 uninfected cells from materials carried by the mycelium to neighbouring infected 

 cells. At any rate it seems to me that in Angiospermic holosaprophytes, the 

 fungus is of the greatest possible significance to the higher plant, and that the 

 latter is dependent upon the fungus for its nitrogenous, and probably for its 

 carbonaceous needs. 



Groom (1895) expresses the opinion that many of the hyphae passing 

 through the sheath are deserting the host, not entering it. This is no doubt true 

 in many cases of the endophytic mycorhiza, the hyphae leaving the host being 

 derived from hyphae which enter a different part of the root; they are probably 

 leaving the host in order to establish another communication between the soil 

 and the endophytic hyphae, and not to act as haustoria for the supply of food 

 to the more superficial hyphae. Groom also believes that many of these hyphae 

 are deserting the host to form spores in the superficial root-cells, especially 

 when they die. , I have seen spore-like bodies in the sheath-cells and outer cortex 

 of Dipodium, and from my observations I have formed the opinion that many 

 hyphae pass outwards from the central cortex into more superficial cells for re- 

 production (Text-fig. 26). 



Is mycorhiza a highly developed and specialised community beneficial to 

 both symbionts, or is the fungais simply a parasite which the host is constantly 

 striving to suppress? These questions seem capable of different answers in 

 different organisms. The entrance of an endophytic fungus into the cells of a 

 host, is largely a matter of chemotropic stimulation; the host-cells contain some- 

 thing which offers a stronger attraction than the medium of the fungus, hence, 

 at first, the endophytic fungus was probably a parasite, absorbing from the host 

 something which it contained in greater abundance than the soil. The entrance 

 of a fungus into a cell produces a reaction in the host's protoplasm, tending to 

 retard or suppress the parasite. This reaction may have led to the exchange 

 of something between fungiis and host-protoplasm, e.g., proteid or nitrogenous 

 matter, and the gradual establishment of a physiological equilibrium between 

 host and endophyte. Gradually therefore, from the condition of pure parasitism, 

 the fungus came to live in symbiotic association with the host-cell, each giving 

 a certain benefit to the other. We know that fungi require carbonaceous foods, 

 and that they are able to obtain readily from humus-soil the poorly oxygenated 

 nitrogen compounds; it seems reasonable therefore to assume that the fungus 

 gives nitrogenous compounds in return for carbonaceous food (starch or sugar) 

 which is obtained more readily from the host than from the soil. 



From this harmonious relationship we pass to the next phase which is shown 

 in Dipodium, namely, the disorganization and digestion of the fungus in certain 

 host-cells, and the yielding of all its proteid to the host-cell. In this case it ap- 



